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.
Upon further inspection `setFromSIMPair` pretty much does what I suggested but also calls `getNetwork`. So I guess my suggestion is:

```js
// This is like {region: 'my-region', carrier: 'my-carrier'}.
var network = getNetwork(pair.mcc, pair.mnc);
// I don't think we actually check carrier currently, maybe leave that out.
if (network.region && network.carrier) {
    // This is setFromSIMPair without the getNetwork call.
    storeNetworkData(network, source);
    break;
}
```