gdk/wayland: Make more matched settings from the portal as valid

gdk/wayland: Make more matched settings from the portal as valid

Commit 97b5fad1315 was a forward port from a gtk3 patch, but the hunk
was applied on the wrong bits of code.

Ensure the initialization paths also do mark settings read from the
portal as valid, so the checks for optional/newer settings actually have
the expected result. It is also desirable to mark settings as valid
after configuration changes (as that patch did effectively do), but not
enough to fix all situations.