background: Clear out dconf if user picks defaults

If a user chooses the default background in control-center, then
they still end up with a change in dconf. This change means that
on upgrades, they'll keep the old distro background configured.

That's probably unexpected and may lead them to believe the upgrade
didn't work.

This commit checks if the user goes back to the distro defaults and
clears out dconf in that case, so that when the disto defaults change
they get the new defaults.