Don't use overlays for WFC error messages and SPN formats

- Error alert and notification messages are stored as string arrays in
  resources. CarrierConfig stores carrier-specific error codes and corresponding
  index which is used to get appropriate messages.
- Supported SPN formats are stored as string arrays in resources. CarrierConfig
  stores index used to get appropriate SPN format.

Bug: 27170754
Change-Id: I7f3635ec0ae73d6190b4b73b11ad13058fc25d54