Lockscreen Notifications: Fix NPE

E/AndroidRuntime( 3778): java.lang.NullPointerException
E/AndroidRuntime( 3778): 	at com.android.keyguard.NotificationViewManager.onScreenTurnedOn(NotificationViewManager.java:286)
E/AndroidRuntime( 3778): 	at com.android.keyguard.KeyguardViewManager.onScreenTurnedOn(KeyguardViewManager.java:825)
E/AndroidRuntime( 3778): 	at com.android.keyguard.KeyguardViewMediator.handleNotifyScreenOn(KeyguardViewMediator.java:1416)
E/AndroidRuntime( 3778): 	at com.android.keyguard.KeyguardViewMediator.access$2300(KeyguardViewMediator.java:104)
E/AndroidRuntime( 3778): 	at com.android.keyguard.KeyguardViewMediator$4.handleMessage(KeyguardViewMediator.java:1131)
E/AndroidRuntime( 3778): 	at android.os.Handler.dispatchMessage(Handler.java:102)

Fix NPE on first boot

Change-Id: I6aa4f62ffbc00be15aa98ee3a137bb0d6add6610