Support sending KEYCODE_HEADSETHOOK KeyEvent from an a11y service

Add a private global action to be made public in S. A service can
trigger the event by calling performGlobalAction(10) in R.

Note: Currently 10 is being used by SystemActions for the Accessibility
shortcut. Update ag/10921262 to avoid a conflict.

Bug: 154038741
Test: Tested with modified TalkBack and Youtube play/pause
Change-Id: I3ca5d25948bfb3eb8520030484cb466ec7188784