Battery Level: Add FLAG_RECEIVER_INCLUDE_BACKGROUND flag to intent

* Use FLAG_RECEIVER_INCLUDE_BACKGROUND flag when sending out
  BluetoothDevice.ACTION_BATTERY_LEVEL_CHANGED so that service apps
  can listen to this at background

Fixes: 127742900
Bug: 127862952
Test: make, unit test
Change-Id: Ied884e771c2c798f5219853395ceec3c7e928161