Allow other modules to change admin page boolean

back_to_top module uses is_adminpage() for the admin page check. This is_adminpage() method checks if route contains admin route context not and based on that returns boolean. But there are situations where user need to add back to top on few admin pages not all.

This can be done by route subscriber to change admin_route option but doing this has its own problem. So there should be a way (alter hook in this case) which allows user to change the boolean return value for admin page.