Transcoder: Added PassthroughTrackTranscoder and unit tests.

PassthroughTrackTranscoder is a MediaTrackTranscoder implementation for passthrough mode.
It manages an internal buffer pool to reuse buffers.
This commit also ensures that it is safe to hold on to dequeued MediaSamples
after the MediaTrackTranscoder is released.

Test: Unit test.
Bug: 152091443
Change-Id: I876957647c5fee0557caf465227ff42cdb0eceee