Only update adaptive mode state on change

Currently we update the adaptive mode state from top to bottom for every state change
even when there hasn't been a change. Change it and only do it once.

Fixes: https://gitlab.gnome.org/GNOME/epiphany/-/issues/1560
Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/985>


(cherry picked from commit 70730e4d)