Fix conflicts in mobile onbarding while rebasing to esr68.2.0

It turns out that there are a number of conflicts between our mobile onboarding code and the coming esr68.2.0 which we need to think about how to resolve. They stem in particular from

https://bugzilla.mozilla.org/show_bug.cgi?id=1570878

and

https://bugzilla.mozilla.org/show_bug.cgi?id=1587631

Not sure whether we want to just back out those changes or make them otherwise compatible with our modifications.