Transcoder: Improve video transcoding with audio passthrough performance

MediaSampleWriter was stalling the video transcoding when processing
chunks of audio samples at a time. Since the underlying muxer
buffers ~1 sec worth of data and interleaves tracks anyway the
sample writer now prioritizes pulling samples from the track that is
farthest behind while making sure the tracks are kept reasonably
in sync, i.e. max ~1 second divergence.

This change also raises the priority of the video transcoding thread
since that will be the bottleneck when audio is pased through.

Test: Unit tests
Fixes: 160268606
Change-Id: I004583b2a31a57882ea543072be321f9f1347508