libstagefright: Use proper ctts offset

Calculate ctts offset as difference between composition
time and decoding time, and avoid adding kMaxCttsOffsetTimeUs
as it increases the sample timestamp by the same amount
which leads to av sync loss for B-frames.

CRs-Fixed: 524739
Change-Id: I75a1bb594536623e3a445af2ee1ceb0cf0926a6e