`SurfaceProducer.Callback` should not fire once the surface is released

[matanlurey]

While it could be annoyed the client now needs to do manual accounting of the state, which defeats the point of this interface.

This caused (part of) #156158 where a dispose-d VideoPlayer still resulted in callback events about onSurfaceDestroyed.

[matanlurey]

Looking at the code, it should already be doing this - so either it's a more subtle bug, or something else.

Will try the following test case:

@Test
public void ImageReaderSurfaceProducerUnsubscribesWhenReleased() {
  // Regression test for https://github.com/flutter/flutter/issues/156434.
  FlutterRenderer flutterRenderer = engineRule.getFlutterEngine().getRenderer();
  TextureRegistry.SurfaceProducer producer = flutterRenderer.createSurfaceProducer();

  // Create and set a mock callback.
  TextureRegistry.SurfaceProducer.Callback callback =
          mock(TextureRegistry.SurfaceProducer.Callback.class);
  producer.setCallback(callback);

  // Release the surface.
  producer.release();

  // Call trim memory.
  ((FlutterRenderer.ImageReaderSurfaceProducer) producer).onTrimMemory(40);

  // Verify was not called.
  verify(callback, never()).onSurfaceDestroyed();
}

[matanlurey]

Surprisingly (to me) the test failed! Iterating on this a bit more.

[matanlurey]

The bug is actually in onTrimMemory, even after a producer is released, it still receives trim memory events!

[matanlurey]

Nope, I'm wrong.

[matanlurey]

Nope, I'm right:

FlutterRendererTest > ImageReaderSurfaceProducerUnsubscribesWhenReleased FAILED
    org.mockito.exceptions.verification.NeverWantedButInvoked: 
    callback.onSurfaceDestroyed();
    Never wanted here:
    -> at io.flutter.embedding.engine.renderer.FlutterRendererTest.ImageReaderSurfaceProducerUnsubscribesWhenReleased(FlutterRendererTest.java:798)
    But invoked here:
    -> at io.flutter.embedding.engine.renderer.FlutterRenderer$ImageReaderSurfaceProducer.onTrimMemory(FlutterRenderer.java:701) with arguments: []
        at io.flutter.embedding.engine.renderer.FlutterRendererTest.ImageReaderSurfaceProducerUnsubscribesWhenReleased(FlutterRendererTest.java:798)