ApnSettings: Exclude MVNO listings that are known not to match

When listing APNs for selection, exclude all of the MVNO entries
of "SPN" type when the SPN doesn't match. This still needs to
be extended for the imsi and gid types, but spn is the majority
of known cases and this helps preventing huge listings for those
MNO that support several MVNO

Addresses CYNGNOS-670

Change-Id: Iea538e84619a96740c65adfac72d32f6a35db082