Add OPUS encoding to the framework

Audio offload support was added to AudioTrack in Q,
but only a few encoding were exposed (MP3 & AAC).

This patch adds only Opus support.
For R we probably want to expose all format that the framework supports.

Bug: 146540917
Test: AudioTrackOffloadTest AudioFormatTest
Change-Id: I2ca57d45081668f76f8d9e153f66d1fd41209685
Signed-off-by: Kevin Rocard <krocard@google.com>