MediaCodec refactoring part 2-a: introduce callback

CodecBase instances do not post reply messages back to MediaCodec
explicitly --- Callback does the job instead.

Bug: 32133435
Test: adb shell am instrument -e size small -w 'android.media.cts/android.support.test.runner.AndroidJUnitRunner'
Test: Play an encrypted content in Play Movies
Change-Id: I210993a3bbc306f970b8e9395a91e8697cfa832e