libbufferstreams: Add the BufferPoolPublisher.

The BufferPoolPublisher submits buffers from a pool over to a
subscriber.

Pair: jshargo
Bug: 296450854, 296101127
Test: atest libbufferstreams-internal_test
Change-Id: Ic473677c9c71b0505c3fcd2b4fb7d0fdf3d7d01b
2 files changed