MediaCodec: update documentation on buffer metadata.

- Deprecated BUFFER_FLAG_SYNC_FRAME (as it actually marks key
  frames).
- Clarified that BUFFER_FLAG_KEY_FRAME is used for encoded output
  buffers.
- Clarified that presentationTimeUs is not an actual render
  timestamp (as opposed to releaseOutputBuffer's render timestamp).

Bug: 12006531
Change-Id: Ia27b9f606a9d7eef9c0c2900cd864a4421c7cf31