[Android] Speed up the method 'FlutterRenderer.getBitmap'

[ColdPaleLight]

The pixel order of ARGB_8888 after storage is ABGR on the Java side. Java is big endian and native is little endian, so the order is RGBA on the C++ side.

So we can use 'Bitmap.copyPixelsFromBuffer' to do pixel copy, which can avoid the conversion of RGBA to BGRA here.

engine/shell/platform/android/platform_view_android_jni_impl.cc

Lines 354 to 362 in d4101ab

	// Our configuration of Skia does not support rendering to the
	// BitmapConfig.ARGB_8888 format expected by android.graphics.Bitmap.
	// Convert from kRGBA_8888 to kBGRA_8888 (equivalent to ARGB_8888).
	for (int i = 0; i < pixels_size; i++) {
	int32_t src_pixel = pixels_src[i];
	uint8_t* src_bytes = reinterpret_cast<uint8_t*>(&src_pixel);
	std::swap(src_bytes[0], src_bytes[2]);
	pixels[i] = src_pixel;
	}

In my local test, the average time consumption of the method FlutterRenderer.getBitmap before optimization is 99.7ms, and it is 66.2ms after optimization.

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[dnfield]

It'd be really nice to have a test of this, perhaps either in flutter_shell_native_unittests or in the scenario_app. Right now AFAICT this code is unused in the engine but it is used in a test in the framework, althogh it's not clear to me whether that test runs on CI or not (https://github.com/flutter/flutter/blob/master/examples/platform_channel/android/app/src/androidTest/java/com/example/platformchannel/ExampleInstrumentedTest.java)

[ColdPaleLight]

It'd be really nice to have a test of this, perhaps either in flutter_shell_native_unittests or in the scenario_app. Right now AFAICT this code is unused in the engine but it is used in a test in the framework, althogh it's not clear to me whether that test runs on CI or not (https://github.com/flutter/flutter/blob/master/examples/platform_channel/android/app/src/androidTest/java/com/example/platformchannel/ExampleInstrumentedTest.java)

@dnfield
The test has been added, However I found an issue related to scenario test flutter/flutter#114805