Transcoder: Add support for pausing transcoding on a sync frame.

- Added support for stopping transcoders on a sync frame.
- Refactored MediaTrackTranscoders and MediaSampleWriter to stop()
asynchronously.
- Fixed callback and error handling logic in MediaTranscoder.
- Added tests for pause and stopping on sync frame.

Bug: 162886306
Test: Unit tests.

Change-Id: If689a10dfee198c674c4c13b865a7c56a901e075