Eliminate the linear cost associated with mSampleSizes.size()

- The linear cost associated with mSampleSizes.size() call causes the
  CPU load to increase overtime as more and more audio/video samples
  are recorded.

- Other Lists used in the MP4 file writer may have similar issues, but
  the size() call is not made for each output audio/video output sample,
  or the size of the list is bounded (not linear to the total number
  of audio/video output samples). As for now, we can live with the small
  cost (tested with 30 minutes long recording).

Change-Id: I23bd93ea4256cb0be0c1649760e39e6809eb4946