Support battery level reporting via Plantronics XEVENT (2/2)

* Add parser for Plantronics AT+XEVENT=BATTERY* AT command
* Record the received battery level information through DeviceProperties
* Add unit test for getBatteryLevelFromXEventVsc() and intent handler

Bug: 35874078
Test: make, test with supporting headsets, unit tests
runtest -c com.android.bluetooth.btservice.RemoteDevicesTest bluetooth
Change-Id: I2972c525a43ff0dc415b63fcbf0f08e7bc0b21a8