workspacesView: Don't tie PgUp/PgDown to mapped state

Both app grid and window picker are now always visible in the overview,
so their handling of the PgUp/PgDown keys conflicts.

Resolve that by checking for the overview state instead.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1798>

js/ui/workspacesView.js

@@ -1142,7 +1142,8 @@ class WorkspacesDisplay extends St.Widget {
     }
 
     _onKeyPressEvent(actor, event) {
-        if (!this.mapped)
+        const { ControlsState } = OverviewControls;
+        if (this._overviewAdjustment.value !== ControlsState.WINDOW_PICKER)
             return Clutter.EVENT_PROPAGATE;
 
         const { workspaceManager } = global;