Ensure USSD codes are sent over IMS when CS is out of service.

They're normally only sent over CS, but if CS is out of service we will
now try them over IMS first.
Also, fixed a bug in ImsPhonecallTracker where a failed USSD code over
IMS would never dismiss the USSD status dialog.

Test: Manual - Connect to WIFI in airplane mode.  Initiate USSD code and
verify it gets to the network.
Bug: 115787581

Change-Id: Ifc79eb283f0e49c5d1a8fe16605c9337c931d38b