mmplugin: Deduplicate code from kcms/cellularnetwork

There is a lot of shared code, let's de-duplicate it

https://invent.kde.org/plasma/plasma-mobile/-/blob/master/components/mmplugin/signalindicator.cpp

https://invent.kde.org/plasma/plasma-mobile/-/blob/master/kcms/cellularnetwork/modem.cpp