Fix VR interaction with screen on/off/lockscreen.

- Add mechanism to prevent VR mode from being enabled.
- Disable VR mode while screen is off, or user is in lockscreen,
  and re-enable when this is no longer the case.

Merged-In: Ia0a7fe8d443a24dce9395053cfe3d1c5847a88f2

Bug: 31665197
Change-Id: Ia0a7fe8d443a24dce9395053cfe3d1c5847a88f2
4 files changed