fixup! Bug 40597: Implement TorSettings module

Bug 41710: Replace StateCallback in TorConnect.

Instead of managing the abstract "State" we directly manager the user
"Stage". We provide backward compatibility with the "State" for android
and about:torconnect. Eventually this logic can be dropped from these
endpoints and they can listen for changes in the "Stage" instead.

The behaviour for about:torconnect is mostly the same as before, with
some exceptions:

1. If the user sees the "Offline" state, and starts and cancels the
   bootstrap, they should return the to the "Offline" state, rather
   than "ConnectToTor".
2. Trying to start a bootstrap via the UI before the settings have
   loaded will do nothing.
3. Pressing a breadcrumb whilst bootstrapping will now also cancel the
   bootstrap.