PowerManager는 Screen을 On/Off할때 WindowManager에 통지를 하고 이는 KeyguardViewMediator를 통해 LockScreen에도 전달된다.
PowerManagerService.mNotificationTask
-> WindowManagerPolicy.screenTurnedOn()
-> KeyguardViewMediator.onScreenTurnedOn();
-> WindowManagerPolicy.screenTurnedOff()
-> KeyguardViewMediator.onScreenTurnedOff();
