Add ACTION_AG_EVENT broadcast when starting VR.

- Before this fix, the HeadsetClientStateMachine may not have
sent a broadcast depending on how the VR was started/stopped.
Resulting in inconsistency and confusion.
- Added a test to cover the usecase.

Bug: 149574523
Test: Unit test + manual
Change-Id: Iecafdc50e780f3f71d9f1c0be35b95da78993d2e