Add support for roaming mobile data run condition

[pkoch]

From https://forum.syncthing.net/t/mobile-data-run-condition-and-roaming/18323/2

As a user who's afraid of paying roaming fees but has an unlimited data plan in their own contry, I'd like to have a run condition that prevents syncthing to work while roaming, in order to not get a problematic data bill.

[pkoch]

I'm happy to take a try at a PR if I'm provided some guidance. :)

[pkoch]

Note to self: look around

syncthing-android/app/src/main/java/com/nutomic/syncthingandroid/service/RunConditionMonitor.java

Line 163 in 3cb96b5

private RunConditionCheckResult decideShouldRun() {

[imsodin]

This seems like a sensible extension to the existing run conditions (if running on mobile data, have another switch to also run on roaming or not).
You already found the relevant part of the code. Feel free to ask any questions here or on a (possibly wip) PR when you are taking a stab at implementing this.

[pkoch]

I've since gotten WiFi in my home, and my motivation to get to this dropped dramatically. Sorry for backing out, and feel free to close if you don't want the clutter.