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.
To avoid hardcoding all possible SPNs for DT in germany. Every SIM with MCC/MNC 262/1 will be DT, except those with "congstar" or "congstar.de" as the SPN.