Parse trex and mehd boxes

Also get duration from mvhd if available, use that in addition to track
durations to determine total length of file, clear track header
struct so we don't read uninitialized garbage from it, and take
composition time offset into account for proper ordering when B frames
are used.

b/15669839

Change-Id: Ibf3e35b5c7299bac11d0e78a391545fc325dd2d0