Listen for device power state for VrMode

ActivityManagerService's sleep state is now dependent on whether or not
there are any awake displays, instead of the device power state. Make
VrManagerService listen to the device power state instead of the
ActtivityManagerService sleep state, since its VirtualDisplay for 2D
activities will keep ActtivityManagerService awake.

Bug: 65605913
Test: manual
Change-Id: Ia620d24977d896e6688020d59850046dd0cb7187