ModemManager 1.18 deprecated mm_location_3gpp_get_mobile_country_code
and mm_location_3gpp_get_mobile_network_code funtions and added
mm_location_3gpp_get_operator_code as a replacement.

This was made because Mobile Network Code is a string where the number
of digits in the value matters, e.g. "10" != "010".

Replace the Mobile Country Code (mcc) and Mobile Network Code (mnc) ints
in GClue3GTower with Operator Code (opc) string. Adapt depending
functions and signals accordingly and add an opc to (mcc, mnc)
conversion function which is used when sending data to the MLS API.