Refactor mobilenetwork.js code (bug 1056088)

https://bugzilla.mozilla.org/show_bug.cgi?id=1056088

Since we are becoming a real packaged app, it's time to clean-up that code.
Can you also mention that this returns the region if we found a match in our "database"? Or update the code so it doesn't mutate? Maybe pull the mutation into another function?

```js
// This is like {region: 'my-region', carrier: 'my-carrier'}.
var carrierData = carrierDataFromSIMPair(pair.mcc, pair.mnc, source);
// I don't think we actually check carrier currently, maybe leave that out.
if (carrierData.region && carrierData.carrier) {
    storeCarrierData(carrierData);
    break;
}
```