Add APIs to get remote device's battery level (1/2)

* Add BluetoothDevice.getBatteryLevel() API to retreive battery level
  information of remote device
* Add BluetoothDevice.ACTION_BATTERY_LEVEL_CHANGED intent to notify user
  that remote device's battery level has changed

Bug: 35874078
Test: make, pair with devices and use them
Change-Id: I41051ee25383f5f3a1e505aef6f8c526385f58bd