Modify AAudio for new AudioTrack callback format

AAudio uses libaudioclient for its 'legacy' pathway.
We refactored libaudioclient callback registration to use an
interface. AudioStreamLegacy now implements the callback since
there will be shared behavior between Record and Track following
refactor.

Change-Id: If07ed3b349c223d7fd55a3e2e18e12a3d0f70865