kcms/mobiledata: Clarify behavior when networkmanager wwanEnabled is off

In NetworkManager, there is a global "wwanEnabled" setting. We do not typically deal with this because toggling it off shuts off cellular in ModemManager (the mobile data toggle just disconnects the connection instead).

Here we ensure that we address the case when "wwanEnabled" is off, and ensure that it's on when mobile data is toggled on. Also remove a check in the SIM page that seemed to depend on it.

Also did some minor refactoring so that the condition is if (enabled) rather than if (!enabled)

I noticed code duplication for between the quicksetting and mobile data kcm as well, opened a ticket to address it here: #309