AudioTrack: support for offloaded playback

Surface new callbacks from native AudioTrack to
  Java through the existing callback method and
  post to the Java instance.
New AudioManager method to query whether offload
  is supported.
Allow using non-PCM format on a track built for
  offload (new option in AudioTrack.Builder)

Numerous "todo" are added for the unhide of the
  API additions and will be in a separate patch.

Bug: 63934228
Test: same as MediaPlayer2 tests

Change-Id: I98a81ffffc95c7596a39ee634dcb89bb854f3178