differentiate between network/ICC values for MCC/MNC; add msisdn

Erm, I don't actually see how we can implement com.nokia.mid.imsi. An IMSI is the combination of an MCC, an MNC, and a [mobile identification number](http://en.wikipedia.org/wiki/Mobile_identification_number), which doesn't appear to be available via any API, not even the certified API [MozMobileICCInfo](https://developer.mozilla.org/en-US/docs/Web/API/MozMobileICCInfo).

Nor does the SystemProperties midlet know what it is on my Asha test device. It lists the value as being "null."