Enable bluetooth a2dpsink to request audio focus from source

- Allow source to initiate play on IoT devices without having audiofocus
  first
- Added mockPackageManager and testSrcPlayIot to A2dpSinkStreamHandler
  unit test

Test: Bluetooth a2dp sink works using the sample-audio-bluetooth
      sample app
Bug: 38098044
Change-Id: I98472c857afa4922ae9ce118c1bcf0f801ffd304