Consolidate WiFi Calling string resources

- move wfcOperatorErrorCodes to CarrierConfig
- corresponding error alert and notification messages are moved to the
  main string.xml and are accessed by index defined in wfcOperatorErrorCodes
- make a list of supported SPN formats and use CarrierConfig to define
  index of appropriate format string

Bug: 27170754
Change-Id: I698908157c1022f47a43ef70f03a571f1d0c75da