Adding "wifi only" option for wifi calling.

- Added "wifi-only" option to the list of wifi calling modes.
- Added code in settings initialization to check whether the carrier
supports including the wifi-only option.  If it does not, changes the
entries/values to a ser which does not include the wifi-only option.

Bug: 27858149
Change-Id: Ib02a97afcf6f2722c28b7a4d9c241774944c52d0