Enable B frame support in MPEG4Writer

This patch allows us to automatically detect whether ctts box is needed in MPEG4Writer.
MPEG4Writer uses ctts version 0 (non-negative offset value) store the composition time
offset on a needed basis.

Currently, the size of the ctts box is not optimized. Optimization will be addressed
in a subsequent patch.

o also changed the private method retrieveDecodingTime(bool) in OMXCodec
  to getDecodingTime()

o related-to-bug: 4232183

Change-Id: Ic6dc7b25ecd258c2506ca4b9c25156e922456e51