Add a framework service tracking VR mode state.

- Implement a "VR mode" that may be enabled by a focused Activity.
- Add a system service that tracks the current VR mode state and notifies
  other core framework services of mode changes.
- Extend NotificationListenerService to allow the bind/unbind lifecycle
  of specified listeners to be triggered by system events.

Bug: 22855417
Bug: 25479708

Change-Id: I1ac8692bbb5521bb6c7cfb9d2b56b98b720f8568