CDNR use ef_pnn as wfc carrier name if spn is null.

1. CDNR should show "registered PLMN name" in Roaming
2. CDNR use ef_pnn as wfc carrier name if spn is null.
3. UE doesn't display operator name if sim does not have ef_spn at
home network.

Bug: 157108864
Test: 1. UE with TMO sim card to test in Roaming and WFC on, and then
check operator name "T-Mobile Wi-Fi Calling". (PASS)
2. UE with taiwan mobile sim card to test at home network, and then
check operator name "TW Mobile". (PASS)
3. UE with TMO sim card to test in Roaming, and then check operator
name "TW Mobile". (PASS)

Change-Id: I1935b76d20c01e0e8c7f2e1c80139b6bd1693bfc