flutter build aar regenerates tooling between each build-mode step (#162705)

Closes https://github.com/flutter/flutter/issues/162703.

I still need to do a similar change for `ios|macos`-`framework` in
https://github.com/flutter/flutter/issues/162704.

I added two unit tests, as well as opted in an integration test to the
flag (it will become the default in
https://github.com/flutter/flutter/pull/160289).

/cc @reidbaker @gmackall.

dev/devicelab/bin/tasks/build_aar_module_test.dart

@@ -21,6 +21,11 @@ Future<void> main() async {
     }
     print('\nUsing JAVA_HOME=$javaHome');
 
+    // TODO(matanlurey): Remove after default.
+    // https://github.com/flutter/flutter/issues/160257
+    section('Opt-in to --explicit-package-dependencies');
+    await flutter('config', options: <String>['--explicit-package-dependencies']);
+
     final Directory tempDir = Directory.systemTemp.createTempSync('flutter_module_test.');
     final Directory projectDir = Directory(path.join(tempDir.path, 'hello'));
     try {