Implemented a MediaHandler that takes text samples and creates
media samples. The data in each media sample is the MP4 box for
non-overlapping cues.
As per WebVtt in Mp4, all cues must be non-overlapping. This handler
takes care of grouping and dividing cues.
Bug: 36138902
Change-Id: I0c1d27964180c14a22cb200591f70e46e04a651f