Support battery level reporting via Plantronics XEVENT (1/2)

* Add related constants to BluetoothHeadset

Bug: 35874078
Test: make, test with supporting headsets
Change-Id: Ied089a065bf9caa0d03f681950fefa2ea990bb3e
Merged-In: Ied089a065bf9caa0d03f681950fefa2ea990bb3e
(cherry picked from commit 0fcbba25b7ac7fd59367c8c9ba3080a3e6ade934)
1 file changed