Support battery level reporting via Apple VSC (2/2)

* Add parsing methods for Apple AT+XAPL and AT+IPHONEACCEV AT commands
* Record the received battery level information through DeviceProperties
* Add unit tests for getBatteryLevelFromAppleBatteryVsc() and intent
  handler

Bug: 35874078
Test: make, test with supporting headsets, unit tests
runtest -c com.android.bluetooth.btservice.RemoteDevicesTest bluetooth
Change-Id: I16de640a53f333f54b5609cbb482d1735a9e5de2