Bug 41394: Implement a setting to always prefer onion sites

Plumb up privacy.prioritizeonions.enabled and expose it to the users.

@dan this is the first time I plumb a Firefox preference to Fenix.

I was a bit worried about having the button not in sync with the preference.

If I understand correctly, we store the preferences in Fenix/Android store, and force Firefox to apply these values to preferences at each initialization of GV.

Fenix part of tor-browser#41394 (closed). See also tor-browser!422 (merged) and android-components!88 (merged).