Fix NPE when the vr manager isn't around at first.

There's a race condition when grabbing the VR service during
ImmersiveModeConfirmation's constructor since the window
manager allocates the ImmersiveModeConfirmation in its own
init(); vr may still be starting up at that time.

Change-Id: Ic0aa0fbf8fd087f01f4690c14e1c68f0670b0bc0
Fixes: 28159168