stagefright: Rework GraphicBufferSource - step 1

- Abstract buffer reference counting so that it does not intimately
assume a BufferQueue. Move releasing acquired buffers into a
callback as this is BQ specific. Having vectors for slots is also
BQ specific.

Use shared, copiable objects to manage buffers and their lifecycles:

CachedBuffer: a cached buffer in a slot

AcquiredBuffer: a buffer acquired from the producer that will
need to be released (and will get released when all copies of
this object go out of scope)

VideoBuffer: an acquired buffer with metadata

- Rename variables to be more descriptive.

- Move associated methods and variables closer together.

- Document methods and some of the operations of GraphicBufferSource.

Bug: 37110143
Change-Id: Iea5e7a376eba2bdd41624a7ddbe1d94c3d0e7b68