Replace ArrayList in ItemContainerGenerator with List<EmptyGroupItem> (…

…#9432)

* replace ArrayList in ItemContainerGenerator with List<EmptyGroupItem>

* use fully qualified type when initializing a new instance

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ItemContainerGenerator.cs

@@ -2055,7 +2055,8 @@ void AddEmptyGroupItem(CollectionViewGroup group)
 
             // add it to the list of placeholder items (this keeps it from being GC'd)
             if (_emptyGroupItems == null)
-                _emptyGroupItems = new ArrayList();
+                _emptyGroupItems = new List<EmptyGroupItem>();
+
             _emptyGroupItems.Add(emptyGroupItem);
         }
 
@@ -2792,7 +2793,7 @@ void OnRefresh()
         private ReadOnlyCollection<object> _itemsReadOnly;
         private GroupStyle      _groupStyle;
         private ItemContainerGenerator _parent;
-        private ArrayList       _emptyGroupItems;
+        private List<EmptyGroupItem> _emptyGroupItems;
         private int             _alternationCount;
 
         private Type            _containerType;     // type of containers on the recycle queue