Add IMS reg checking before display Cross Sim SPN

Add a condition to check if IMS have registered before displaying Cross
Sim SPN. Without this the Cross Sim SPN will be displayed if radioTech
is CrossSim no matter is  IMS is still registering. It may confuse User
on why IMS entered Backup Calling when other RAT is available.

Flag: EXEMPT bugfix
Bug: 302469576
Test: atest
Change-Id: I0c841e432da47394bb3f14e76a0e4d996883946c