Always show emergency notification while UE is registered over WFC

The emergency notification should be shown if the UE is registered over
WFC regardless of whether the UE is in the cellular service area or
not.
Furthermore, the notification should not be canceled by user operation.

Test: auto - Passed CarrierServiceStateTrackerTest
Test: manual - Checked that the notification will always be shown while
UE is registered over WFC.
Bug: 138622099

Change-Id: Id3cdbe28e95b5b8c662c8799a0930a37c91a417e