Fix NPE in BaseStatusbar in onListenerConnected

If NotificationListenerService is not bound the
getActiveNotifications function will return null.
This will result in a Nullpointer exception in BaseStatusbar
onListenerConnected if it is called. A Nullpointer check was
added in onListenerConnected to avoid this Nullpointer exception.

Change-Id: I0aec040f11101e8f7b5863879b3774dc2bb6ce2b