notificationDaemon: Namespace private sender-pid hint

The hint is a private implementation detail between the public
and internal services, not something anybody else should set
(*cough* libnotify *cough*).

Prefix the name to hopefully make that clearer.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3204>

js/ui/notificationDaemon.js

@@ -162,7 +162,7 @@ class FdoNotificationDaemon {
         this._notifications[id] = ndata;
 
         let sender = invocation.get_sender();
-        let pid = hints['sender-pid'];
+        const pid = hints['x-shell-sender-pid'];
 
         let source = this._getSource(appName, pid, ndata, sender, null);
         this._notifyForSource(source, ndata);