Draft: Base ui changes 04
Closes #34403 (closed) #40015 (closed) #40016 (closed)
#34403 (closed) 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 (moved), 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.