Transcoder: Refactor sample writer to not block clients.

This commit fixes an issue with hangs in the transcoder
by not letting samples from all tracks go directly to the
backing muxer. This relies on tracks being synchronized by
the sample reader and that the muxer buffers and interleaves
samples internally.

Test: Transcoder unit tests.
Fixes: 165374867
Change-Id: I99d2dbfa4eb094b7364848a1a8aa3d3d8742140d