cleanup: Always use single quotes

We have made good progress with transitioning to the new style,
to the point where we can complete it with a final push.

Start with changing the remaining places that still use double
quotes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2866>

js/ui/barLevel.js

@@ -227,6 +227,6 @@ export const BarLevel = GObject.registerClass({
     }
 
     _valueChanged() {
-        this._customAccessible.notify("accessible-value");
+        this._customAccessible.notify('accessible-value');
     }
 });