PowerUI: dismiss warning notifications when falling back.

The full-screen intents simply fire broadcasts to open fallback
dialogs.  This means the warning notifications are left visible
under the dialog on the lockscreen, or after the dialog is
quickly dismissed.

Once we launch the fallback dialogs, there is no reason for the
notification form to exist.

Bug:13329308
Change-Id: I5735c65d7e8eb620493d3ef3a2cfc27f75cecbb5