Persistent 'emergency calls' banner in the notification panel.

While carrier info (or SSID for devices without mobile data)
will still be shown in a floating text view at the bottom of
the notification shade, emergency-calls-only mode will now
be shown in the notification panel header so that it cannot
be obscured by notifications.

Change-Id: I714b6801be2b9b631b86b51d229440445eff5e76