SDK / exoplayer

1. Have frame buffer callbacks return Libgav1StatusCode instead of int.
The 0 (success), -1 (failure) return value convention is less obvious.
Note: The callers of frame buffer callbacks,
BufferPool::OnFrameBufferSizeChanged() and YuvBuffer::Realloc(),
currently return bool, so more work is needed to propagate the frame
buffer callbacks' Libgav1StatusCode return value to the Decoder API.

2. Allow the FrameBufferSizeChangedCallback to be omitted if the frame
buffer size information is not useful to the application.

3. Remove the old (version 1) frame buffer callback API. Remove the
frame buffer callback adaptor.

frame_buffer2.h is renamed frame_buffer.h.
Libgav1FrameBuffer2 is renamed Libgav1FrameBuffer.
GetFrameBufferCallback2 and ReleaseFrameBufferCallback2 are renamed
GetFrameBufferCallback and ReleaseFrameBufferCallback.

PiperOrigin-RevId: 295971183