NotificationMgr: Only do DOS protection if not updating a notification.

  Only check for DOS protection if the incoming id being sent by the callee is
  unknown to us. Otherwise, you can post MAX_PACKAGE_NOTIFICATIONS and never
  be able to update any of them without first cancelling an existing notification.

Change-Id: Ideab9a3d01a45e15942db63864f964b3a3308611