At API Version 29+, we use ImageReader instead of SurfaceTexture.

However, many devices will continue to use the older SurfaceTexture-based APIs for a long time, and we've historically broken external texture support here.

My suggestion is to:

1. Add a Skia variant with API version 28 to ci/builders/linux_android_emulator.json
2. Do the same thing for Impeller + OpenGLES
3. Migrate the "plugins" in scenario_app/android/* (example) to the SurfaceProducer pattern

With these steps, we'll be running:

1. Skia + OpenGLES (Implicit) + ImageReader
2. Skia + OpenGLES (Implicit) + SurfaceTexture
3. Impeller + OpenGLES (Explicit) + ImageReader
4. Impeller + OpenGLES (Explicit) + SurfaceTexture
5. Impeller + Vulkan (Explicit) + ImageReader

/cc @johnmccutchan @gaaclarke @jonahwilliams for FYI, I'll be sending a PR but chime in if you disagree.