Transcoder: Improve AV transcoding speed by enforcing sequential sample access.

MediaSampleReader was bottlenecking the transcoding pipeline due to
non-sequential sample access. This commit adds an option to the sample
reader to enforce sequential sample access by blocking reads until
the underlying extractor advances to that specific track.
Follow-up: b/165374867 Make MediaSampleWriter robust against buffering track transcoders

Fixes: 160268606
Test: Transcoder unit tests, and benchmark tests.
Change-Id: Id2a363d06df927ea3e547462c52803594e0511e1