UE shows ef_pnn as WFC carreir name if spn is null.

If spn is null, UE shows ef_pnn as WFC carrier name. It controls by
carrier config KEY_WFC_CARRIER_NAME_OVERRIDE_BY_PNN_BOOL.

Bug: 157108864
Test: UE with TMO sim card to test in Roaming and WFC on, and then
check operator name. (PASS)

Change-Id: I05151118e5e418962b8877a13054b0c29f92568c
Merged-In: I05151118e5e418962b8877a13054b0c29f92568c