Moved FaceLock bind/unbind calls inside of null check

- I am not sure under what circumstances mKeyguardView can be null in
  onScreenTurnedOn() and I never saw this behavior before the commit,
  but it can happen and prevent the device from booting

- Patched to fix line length

Change-Id: I39efa5c1d68158af5c108430036fe7c715ef855b