Cleanup API docs for audio codecs and add intdef.

- Cleanup the API docs for the audio codec constants.
- Add an IntDef for the constants
- Refer to IntDef in the EXTRA_AUDIO_CODEC definition.
- Cleanup API docs for EXTRA_AUDIO_CODEC to make it more clear that
the codecs mentioned are not the only ones supported.

Fixes: 146563762
Test: Compile/make update-api
Change-Id: If837c640033c76f32dfa25dcf9c7d1d6cb344d7b