Implement opacity FlutterMutator for hcpp (#164147)

This is essentially a reland of
https://github.com/flutter/engine/pull/30264/, except that it only
applies the opacity if the new alpha (alpha = 255*opacity, where
opacity∈[0,1]) is different from the current alpha.

Also adds a new opacity screenshot test. Need to figure out how to bring
up a new screenshot test... does the screenshot get uploaded
automatically? Do I need to manually upload somewhere?

Fixes https://github.com/flutter/flutter/issues/164212

## Pre-launch Checklist

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there for submitting PRs.
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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

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---------

Co-authored-by: Gray Mackall <mackall@google.com>

dev/integration_tests/android_engine_test/lib/hcpp/platform_view_opacity_main.dart

@@ -0,0 +1,117 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+
+import 'package:android_driver_extensions/extension.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_driver/driver_extension.dart';
+
+import '../src/allow_list_devices.dart';
+
+void main() async {
+  ensureAndroidDevice();
+  enableFlutterDriverExtension(
+    handler: (String? command) async {
+      return json.encode(<String, Object?>{
+        'supported': await HybridAndroidViewController.checkIfSupported(),
+      });
+    },
+    commands: <CommandExtension>[nativeDriverCommands],
+  );
+
+  // Run on full screen.
+  await SystemChrome.setEnabledSystemUIMode(SystemUiMode.immersive);
+  runApp(const _OpacityWrappedMainApp());
+}
+
+final class _OpacityWrappedMainApp extends StatefulWidget {
+  const _OpacityWrappedMainApp();
+
+  @override
+  State<_OpacityWrappedMainApp> createState() {
+    return _OpacityWrappedMainAppState();
+  }
+}
+
+class _OpacityWrappedMainAppState extends State<_OpacityWrappedMainApp> {
+  double opacity = 0.3;
+
+  void _toggleOpacity() {
+    setState(() {
+      if (opacity == 1) {
+        opacity = 0.3;
+      } else {
+        opacity = 1;
+      }
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Opacity(
+        opacity: opacity,
+        child: ColoredBox(
+          color: Colors.white,
+          child: Stack(
+            alignment: Alignment.center,
+            children: <Widget>[
+              TextButton(
+                key: const ValueKey<String>('ToggleOpacity'),
+                onPressed: _toggleOpacity,
+                child: const SizedBox(
+                  width: 300,
+                  height: 300,
+                  child: ColoredBox(color: Colors.green),
+                ),
+              ),
+              const SizedBox(
+                width: 200,
+                height: 200,
+                child: _HybridCompositionAndroidPlatformView(
+                  viewType: 'changing_color_button_platform_view',
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+final class _HybridCompositionAndroidPlatformView extends StatelessWidget {
+  const _HybridCompositionAndroidPlatformView({required this.viewType});
+
+  final String viewType;
+
+  @override
+  Widget build(BuildContext context) {
+    return PlatformViewLink(
+      viewType: viewType,
+      surfaceFactory: (BuildContext context, PlatformViewController controller) {
+        return AndroidViewSurface(
+          controller: controller as AndroidViewController,
+          gestureRecognizers: const <Factory<OneSequenceGestureRecognizer>>{},
+          hitTestBehavior: PlatformViewHitTestBehavior.transparent,
+        );
+      },
+      onCreatePlatformView: (PlatformViewCreationParams params) {
+        return PlatformViewsService.initHybridAndroidView(
+            id: params.id,
+            viewType: viewType,
+            layoutDirection: TextDirection.ltr,
+            creationParamsCodec: const StandardMessageCodec(),
+          )
+          ..addOnPlatformViewCreatedListener(params.onPlatformViewCreated)
+          ..create();
+      },
+    );
+  }
+}

dev/integration_tests/android_engine_test/test_driver/hcpp/platform_view_opacity_main_test.dart

@@ -0,0 +1,64 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+
+import 'package:android_driver_extensions/native_driver.dart';
+import 'package:android_driver_extensions/skia_gold.dart';
+import 'package:flutter_driver/flutter_driver.dart';
+import 'package:test/test.dart';
+
+import '../_luci_skia_gold_prelude.dart';
+
+/// For local debugging, a (local) golden-file is required as a baseline:
+///
+/// ```sh
+/// # Checkout HEAD, i.e. *before* changes you want to test.
+/// UPDATE_GOLDENS=1 flutter drive lib/hcpp/platform_view_opacity_main.dart
+///
+/// # Make your changes.
+///
+/// # Run the test against baseline.
+/// flutter drive lib/hcpp/platform_view_opacity_main.dart
+/// ```
+///
+/// For a convenient way to deflake a test, see `tool/deflake.dart`.
+void main() async {
+  const String goldenPrefix = 'hybrid_composition_pp_platform_view';
+
+  late final FlutterDriver flutterDriver;
+  late final NativeDriver nativeDriver;
+
+  setUpAll(() async {
+    if (isLuci) {
+      await enableSkiaGoldComparator(namePrefix: 'android_engine_test$goldenVariant');
+    }
+    flutterDriver = await FlutterDriver.connect();
+    nativeDriver = await AndroidNativeDriver.connect(flutterDriver);
+    await nativeDriver.configureForScreenshotTesting();
+    await flutterDriver.waitUntilFirstFrameRasterized();
+  });
+
+  tearDownAll(() async {
+    await nativeDriver.close();
+    await flutterDriver.close();
+  });
+
+  test('verify that HCPP is supported and enabled', () async {
+    final Map<String, Object?> response =
+        json.decode(await flutterDriver.requestData('')) as Map<String, Object?>;
+
+    expect(response['supported'], true);
+  }, timeout: Timeout.none);
+
+  test('should screenshot a rectangle with specified opacity', () async {
+    await expectLater(nativeDriver.screenshot(), matchesGoldenFile('$goldenPrefix.opacity.png'));
+  }, timeout: Timeout.none);
+
+  test('should start with opacity, and toggle to no opacity', () async {
+    await expectLater(nativeDriver.screenshot(), matchesGoldenFile('$goldenPrefix.opacity.png'));
+    await flutterDriver.tap(find.byValueKey('ToggleOpacity'));
+    await expectLater(nativeDriver.screenshot(), matchesGoldenFile('$goldenPrefix.no_opacity.png'));
+  }, timeout: Timeout.none);
+}