Finish up B frame support in MPEG4Writer

o optimize to reduce the size of the size of the ctts box
o change the type for the time offset field in ctts table entry from int32_t to uint32_t according to the mp4 file spec
o also moved away from MediaDebug and used ADebug instead.

o related-to-bug: 4232183

Change-Id: I19364303728da64359c63169eec7487508c1d0f8