Add a VR listener service.

Bug: 22855417
Bug: 26724891
Bug: 27364145

- Add an API for VrListenerService, which is bound/unbound
  from the framework when the system VR mode changes.
- Allow only a single bound VrListenerService at a time.
- Monitor allowed VrListenerService implementations from
  VrManagerService and evict services as needed when packages,
  users, or settings change.
- Remove previous VR functionality in NotificationListenerService.
- Add component target to Activity#setVrMode to allow
  explicit selection of the running VrListenerService from
  the current VR activity.

Change-Id: I776335f4441be0e793d3126f2d16faf86a8c621a