Make sure torbutton is initialized properly in Fenix

Moving from tor-browser#40098 (closed).

Focusing on the security prefs and domain isolator features and leaving out "New identity" and "Circuit display", as they probable need more changes to work. We can do those when we get to implement these features.

The domain isolator is already working, since it's initialized as a service. For the security prefs, the noscript communication is working but we need to do changes to initialize the security-prefs.js module. My idea is to move the current initialization from torbutton.js to the existing startup-observer.js so that it's initialized on startup both for mobile and desktop. Since for now we should not need anything else from torbutton.js for mobile, that lets us avoid having to do a browser patch to initialize torbutton.js for geckoview.

The FP and disk pref syncing done in torbutton.js I think we can avoid for mobile, at least in the beginning if we don't support a "non-private mode".