Allows the authoring engine to skip frame.

This is 1st part of the work to allow audio and video resync if
we found out that audio and video are out of sync during authoring

- also fixed a problem in AACEncoder::read() where the buffer acquired
  from the buffer group does not release when error out at
  reading from source.

Change-Id: I8a2740097fcfdf85e6178869afeb9f3687a99118