Transcoder: Added MediaTrackTranscoder and VideoTrackTranscoder

A track transcoder transcodes samples from a single track of a media file.
MediaTrackTranscoder is the base class for all track transcoder, and
VideoTrackTranscoder is a concrete implementation for video tracks.

Test: Unit test.
Bug: 152091443
Change-Id: I7980a3cda40229004b6bca5212d3e903f19a3017