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

* Add backend service methods for BluetoothDevice.getBatteryLevel()
* Add battery level field in DeviceProperties with getters and setters
* Add updateBatteryLevel() method in RemoteDevices
* Add resetBatteryLevel() method in RemoteDevices
* Reset battery level for device when device is disconnected in
  aclStateChangeCallback() to ensure a BATTERY_LEVEL_CHANGED intent
  when device first report battery level information after connection
* Add tests for updateBatteryLevel() and resetBatteryLevel()

Bug: 35874078
Test: make, pair with devices and use them, unit tests
runtest -c com.android.bluetooth.btservice.RemoteDevicesTest bluetooth
Change-Id: Id770ee08b3cd295563660557b9a1bf0e643b32b8