[fenix] For https://github.com/mozilla-mobile/fenix/issues/27016 - Ensure smooth search UX after the MR onboarding is closed
If the app is opened from the search widget and the MR onboarding is shown then
the backstack will have the following structure:
- root, homeFragment, searchDialogFragment, onboardingFragment
as opposed to otherwise
- root, homeFragment, searchDialogFragment.
This patch allows to avoid the MR onboarding fragment causing the
SearchDialogFragment to not know that below it is the HomeFragment and
consequently not applying transparency or propagate user touches to the parent
Activity.