backends: Update two-finger scrolling before edge scrolling

As whether edge scrolling is enabled depends on whether two-finger
scrolling is disabled, make sure to update two-finger scrolling first.

Note that this only fixes the problem on startup. Changing the
settings in GSettings directly might cause an inconsistent state, but
the main UI for this setting, gnome-control-center, makes sure to
update two-finger scrolling before edge scrolling.

https://bugzilla.gnome.org/show_bug.cgi?id=769276