universal-access: only store current icon them if not HighContrast activated

The panel tries to keep track of the icon theme in use when HighContrast isn't. For that, it stores in self->old_icon_theme. That is, however, always updated when org.gnome.desktop.interface is touched, regardless of whether HighContrast is already turned on.

Thus, it makes sense to check whether HighContrast is selected in org.gnome.desktop.a11y.interface, and only store the old_icon_theme under the condition high-contrast wasn't selected.

Note: While this fixes #2640 (closed), the underlying mechanic is somewhat broken by design, as it relies on the idea that Settings was opened with high-contrast turned off. If it is closed and between turning it on and wanting to turn it off again (more specifically: CcUaSeeingPage deallicated), the previous themes cannot be determined anymore.