libbufferstreams: Add Buffer, BufferPool and BufferOwner.

This change adds supoprt for a simple buffer pool. Buffer pools can be
notified when a buffer is dropped, so the buffer can be provided by the
pool again.

We introduced the concept of a BufferOwner, which is generic and can be
implemented by a client for their own custom buffer pools.

Along the way we updated the Frame struct to use a Buffer instead of a
AHardwareBuffer.

Pair: jshargo
Bug: 296450854, 296101127
Test: atest libbufferstreams-internal_test
Change-Id: Ib7c1ba19f96d1deb3d329366aa9215ad89228f9e