Unhide audio offload for android.media.AudioTrack

Unhide some compressed audio formats for offloading
  or direct output.
Unhide AudioManager method to query whether a given
  format can be offloaded on the device.
Unhide AudioTrack.Builder method to request offload.
Unhide callback for new AudioTrack events and
  associated methods.

Bug: 63934228
Test: same as MediaPlayer2 tests
Change-Id: If763eb2424baa6aa55cb60e96bb3564c4234e23f