Support battery level reporting via Apple VSC (1/2)

* Add related constants to BluetoothHeadset

Bug: 35874078
Test: make, test with supporting headsets
Change-Id: I9f49ea28efdf6de0e751a377d879fe13a47655d5
Merged-In: I9f49ea28efdf6de0e751a377d879fe13a47655d5
(cherry picked from commit 679d0bc8f99145d70af0c8006d807b55110fe3b5)
1 file changed