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.

Test: Fix merge from nyc-mr2-dev.

Bug: 31665197
Bug: 33655854
Change-Id: If777e0ef40b29bff42722f2aa10cff86a5831bc0