Move .cxx directory out of android/app (#166277)

This is a build artifact, so move to `build/app/intermediates/flutter`.

See/ fixes https://github.com/flutter/flutter/issues/160372

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---------

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

dev/devicelab/bin/tasks/gradle_plugin_fat_apk_test.dart

@@ -152,6 +152,27 @@ Future<void> main() async {
             throw TaskResult.failure("Shared library doesn't exist");
           }
         }
+
+        section('AGP cxx build artifacts');
+
+        final String defaultPath = path.join(project.rootPath, 'android', 'app', '.cxx');
+
+        final String modifiedPath = path.join(
+          project.rootPath,
+          'build',
+          'app',
+          'intermediates',
+          'flutter',
+          '.cxx',
+        );
+        if (Directory(defaultPath).existsSync()) {
+          throw TaskResult.failure('Producing unexpected build artifacts in $defaultPath');
+        }
+        if (!Directory(modifiedPath).existsSync()) {
+          throw TaskResult.failure(
+            'Not producing external native build output directory in $modifiedPath',
+          );
+        }
       });
 
       return TaskResult.success(null);