Although H2BGraphicBufferProducer is only used by C2VdaBqBlockPool,
the logic of H2BGraphicBufferProducer is not highly coupled. This CL
splits the class to a dedicated file.

Bug: 160110407
Test: mmm external/v4l2_codec2
Change-Id: Icec2d189fe9df832d71aeb7b0f25ec22df8cc2d0