Offload support for android.media.AudioTrack

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.
Implement registration and callback for stream events.
JNI: configure native AudioTrack for offload.
Remove unused imports.

Bug 113352077
Bug 86837964
Test: "make offline-sdk-docs" and "adb shell clof --source /sdcard/Music/my.mp3"

Change-Id: I565668913076a57ea2c444f1f9268b21024ccde0