Blocks #156488.

In #156488, we wanted to simplify how ImageReaderSurfaceProducer works, and, for external texture plugins, stop listening to TRIM_MEMORY Android events as a determination to invalidate the surface (and rebuild). A blocker to the approach is some incompatible logic in how the C++ engine works with ImageReader.

In a bit more detail, the following stanza conditionally draws the texture image if available:

However, in practice, we see that, if the following occurs:

1. An ImageReaderSurfaceProducer is created and drawn to (statically, i.e. not streaming an image from another source)
2. You background the app, triggering OnGrContextDestroyed
3. You resume the app, we no longer have a dl_image_, but nothing will trigger a new image to become available

... we (at HEAD), hit No DlImage available for ImageExternalTexture to paint.

This can be observed in android_engine_test with the following patch:

# engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/renderer/FlutterRenderer.java#L447
-  private static final boolean CLEANUP_ON_MEMORY_PRESSURE = true;
+  private static final boolean CLEANUP_ON_MEMORY_PRESSURE = false;

... and then run an integration test:

cd dev/integration_tests/android_engine_test
et run lib/external_texture/surface_producer_smiley_face_main.dart -c android_debug_unopt_arm64

... and using the home button on a device (or emulator), switching to another app, and switching back.

A "fix" (that is not proper) is the following:

diff --git a/engine/src/flutter/shell/platform/android/image_external_texture.cc b/engine/src/flutter/shell/platform/android/image_external_texture.cc
index ed9e7b3c7f9..60bc7d20c9c 100644
--- a/engine/src/flutter/shell/platform/android/image_external_texture.cc
+++ b/engine/src/flutter/shell/platform/android/image_external_texture.cc
@@ -66,7 +66,7 @@ void ImageExternalTexture::OnGrContextCreated() {
 // Implementing flutter::ContextListener.
 void ImageExternalTexture::OnGrContextDestroyed() {
   if (state_ == AttachmentState::kAttached) {
-    dl_image_.reset();
+    // dl_image_.reset();
     image_lru_.Clear();
     Detach();
   }

What we should do is instead, is if we fail to acquire the latest image, check the previous image instead:

Ideally this is done where it impacts all subclasses of ImageExternalTexture (this is not specific to Vulkan).