keyboard: Fix display keyboard layout crash with some input methods

Some ibus input method will use the current keyboard layout,
and return "default" as the keyboard layout.
When this happens, display the current keyboard layout instead.

Fixes #3863.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1756>

js/misc/keyboardManager.js

@@ -156,4 +156,8 @@ var KeyboardManager = class {
         let options = this._xkbOptions.join(',');
         return options;
     }
+
+    get currentLayout() {
+        return this._current;
+    }
 };