Refactor VR state management in ActivityManagerService.

- Move VR-related state from ActivityManagerService into
  VrController.
- Move VR-related render thread scheduling logic from
  ActivityManagerService into VrController.
- Update state logic to better enforce invariants.

Bug: 36214284

Test: Covered by existing tests.
Change-Id: I02d1488da0c0f65a10dd0369f38f1a0c47cee174
2 files changed