SST: Update SPN display on IWLAN reg tech change

- When IMS registration tech moves from  "regular" IWLAN (WiFi AP)
to CROSS_SIM, the ServiceState objects do not change, so it does not
trigger an update to the carrier alpha tag.
- SST will now manually update alpha tag when IMS service updates the IMS
registration state.
- go/pag/2315643 for the ShannonIMS svc change.

Bug:249609782
Test: Live test on P10: connect to WiFi AP when 1 sub is in Backup
Calling, observe UI update from '<Carrier> Backup Calling' to
'<Carrier> WiFi Calling'.

Change-Id: If605b8bdb3d44d61b3588a8ce35d4c4d9bc00d8f