Remake settings using AndroidX library

https://developer.android.com/guide/topics/ui/settings.

The old `android.preference`-based implementation [is deprecated](https://developer.android.com/reference/android/preference/Preference) in Android 10.

As far as I know, the AndroidX-based implementation is not a drop-in replacement, so some extra work is expected.

Maybe scavenge something from #820.