light-style: Always save scheme preference on enable()

Disable() should restore the scheme preference that was used when
the extension was enabled, not when it was first initialized.

Even if it's unlikely to be relevant in practice, let's make sure
we save the correct state.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/257>

extensions/light-style/extension.js

@@ -21,16 +21,13 @@ const {St} = imports.gi;
 const Main = imports.ui.main;
 
 class Extension {
-    constructor() {
-        this._savedColorScheme = Main.sessionMode.colorScheme;
-    }
-
     _updateColorScheme(scheme) {
         Main.sessionMode.colorScheme = scheme;
         St.Settings.get().notify('color-scheme');
     }
 
     enable() {
+        this._savedColorScheme = Main.sessionMode.colorScheme;
         this._updateColorScheme('prefer-light');
     }