DO NOT MERGE Enhance the NotificationListenerService api.

Add to NotificationListenerService:
 - getActiveNotificationKeys() : String[]
 - getActiveNotifications(String[] keys) : StatusBarNotification[]
 - cancelNotifications(String[] keys) : void
 - onListenerConnected(String[] keys) : void

Add to StatusBarNotification:
 - getKey() : String

Bug:13562828
Change-Id: I9da222cb4af22864bc0ecdecc3f121bced0723d6