userWidget: Stop expand flags from propagating

Since commit 9c2da01a9, the avatar's child is expanded to account
for the new St.Bin behavior. However as expand flags are propagated
up, this now results in avatar actor getting unintentionally expanded
in places like the end-session dialog.

Stop this by explicitly setting the avatar actor to not expand.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3221>

js/ui/userWidget.js

@@ -31,6 +31,8 @@ class Avatar extends St.Bin {
             reactive: params.reactive,
             width: params.iconSize * themeContext.scaleFactor,
             height: params.iconSize * themeContext.scaleFactor,
+            x_expand: false,
+            y_expand: false,
         });
 
         this._iconSize = params.iconSize;