Use "primary provider" information from MBPI

Fixes MBPI selections where no SPN is given, and an MVNO is accidentally picked instead of the network's actual owner. Takes the "primary" field of MBPI records into account, see https://git.gnome.org/browse/mobile-broadband-provider-info/tree/serviceproviders.2.dtd#n9 . Note that the "primary" field is currently not set for very many providers in the database.