Detect MVNO using the SPN (bug 1049908)

https://bugzilla.mozilla.org/show_bug.cgi?id=1049908

Tested with a regular DT SIM and a Congstar SIM, it works fine, I get `deutsche_telekom` and `congstar` as a carrier, respectively.
We could test a few others, but the important thing is that the code handling mozMobileConnection works correctly, not the specific MCC/MNC -> region/carrier combination.