Enable CMAS alert tone and alert dialog for AMBER alerts.

The CMAS UI did not play the CMAS alert tone for AMBER alerts,
showing them as a regular notification instead. Carrier requirements
state that the CMAS alert tone should play for all CMAS alerts.
Remove the AMBER alert exception from
CellBroadcastMessage.isEmergencyAlertMessage().

Bug: 7417676
Change-Id: If24f6b4ebd4633f671beebbe7784b6cc012c358a