Use audio clock as the reference media clock

o Only do this for realtime applications
o Adjust other track clock based on audio clock
o Assume other track uses wall clock as the media clock
o Use some heuristics to reduce the size of stts box by 2/3.

- also
o Remove one unused key from MetaData.h

Change-Id: Ib9432842627b61795b533508158c25258a527332