Closes #34403 #40015 #40016
#34403 is the largest change. It retains some of the logic from !6 (closed) but the UI is not present anymore, so the browsing mode cannot change.
android-components#40001, tor-browser-build#40074 (closed) and tor-browser-build#40075 (closed) will have a separate MR.
Note, this branch is still based on v80.0b6. I'm hoping this will make reviewing easier. I'll create another based on 81.1.0b1.