authPrompt: Grab key focus when making entry sensitive

We currently let the entry of the autoPrompt grab the key focus inside
setQuestion(), which is called from _onAskQuestion(), which is the
callback of the "ask-question" signal.

It seems that the "ask-question" signal isn't emitted again right after
the password-check failed, but a few seconds after that. Since we get
the "verification-failed" signal earlier than "ask-question" (right
after we know the check failed) and we also get a hint whether the entry
should be usable again with the canRetry argument, we can also grab key
focus to in the same step.

So do that by grabbing key focus when making the entry sensitive.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2655

js/gdm/authPrompt.js

@@ -424,7 +424,13 @@ var AuthPrompt = GObject.registerClass({
     }
 
     updateSensitivity(sensitive) {
+        if (this._entry.reactive === sensitive)
+            return;
+
         this._entry.reactive = sensitive;
+
+        if (sensitive)
+            this._entry.grab_key_focus();
     }
 
     vfunc_hide() {