Fix to update wfc mode in roaming network

CarrierConfigManager may be null while device is booting, so ImsManager.setWfcMode() will not be called. However, the last cached roaming state is still changed. Even if ImsPhone receives the roaming state update again and CarrierConfigManager becomes available, ImsManager.setWfcMode() will not be called again because the cached roaming state already reflects the change.

Bug: 317298331
Test: atest ImsPhoneTest
Test: manul test (Panther, AT&T SIM)
      changed carrier config editable_wfc_roaming_mode_bool as true
      enabled screen PIN lock
      changed WFC prefered mode
      Checked text based log after device reboot
Change-Id: I632b195d4f8a25d4aeb5322b20fcafa6980ae60c