prepend MCC value to com.nokia.mid.mnc

Here's a first take at making com.nokia.mid.mnc look like what it looks like on my Asha test device.

One thing that isn't clear to me is what to do about one- and two-digit MNC values. According to [this Wikipedia page](http://en.wikipedia.org/wiki/Mobile_country_code), there are two digit MNCs like 01 and 10, and "MNC of 001 is not the same as MNC of 01". But [MozMobileNetworkInfo](https://developer.mozilla.org/en-US/docs/Web/API/MozMobileNetworkInfo) represents MCC and MNC values as shorts, which means it'll stringify them to 1 and 10.

So does Nokia zero-pad these values? I can't tell on my Asha test device, whose MCC and MNC values are 310 and 410, respectively (so com.nokia.mid.mnc is "310410"). (I didn't see any two-digit country codes, so I think the issue only concerns network codes.)