Make TorSettings interact with the old Android Settings

We want to re-use desktop's connection assist, which is integrated with TorSettings.

TorSettings uses Firefox prefs, but on Android we're currently using something native that Android provides.

We need to decide which mechanism we want to keep using and write some plumbing (and even a migration procedure, if we end up using prefs, even though Mozilla seems to prefer the native Android one, which is something to consider).