Fix hiding of sign-in to network notification.

Don't rely on the NetworkAgent still being connected when we go to
hide the notification.  The notification is hidden when the
NetworkAgent is disconnected so that wasn't a safe assumption.
By using the NetID as the notification identifier we also fix the
issue of multiple notifications of the same network type inadvertently
hiding the incorrect notification with the same network type.

bug:16317917
Change-Id: I01fdc466a0f430af9fc378445586ec7b83b3ac83