use "mobilenetwork" to access privileged subset of Mobile Connection API

The Mobile Connection API is confusing: in theory, *MozMobileNetworkInfo* is available to privileged apps, but the only way to access it is through the *voice.network* and *data.network* properties and the *getNetworks()* method of a *MozMobileConnection* object, which are only available to certified apps.

But there are two properties of *MozMobileConnection*, *lastKnownNetwork* and *lastKnownHomeNetwork*, which are not documented on MDN but are referenced in the [MozMobileConnection WebIDL file](http://mxr.mozilla.org/mozilla-central/source/dom/webidl/MozMobileConnection.webidl?rev=b5cecf9d2750#69), that provide the information our *mobileInfo* module needs: the MCC and MNC for the network and ICC (SIM card).

Here's an update to *mobileInfo* that uses those properties to get that info.