Make utility methods non-virtual.

Not sure why these method were virtual anyway: any customization should be done by overriding the non-generic `CreateItemContainerGenerator` method.

src/Avalonia.Controls/TreeView.cs

@@ -401,7 +401,7 @@ namespace Avalonia.Controls
         protected virtual ITreeItemContainerGenerator CreateTreeItemContainerGenerator() =>
             CreateTreeItemContainerGenerator<TreeViewItem>();
 
-        protected virtual ITreeItemContainerGenerator CreateTreeItemContainerGenerator<TVItem>() where TVItem: TreeViewItem, new()
+        protected ITreeItemContainerGenerator CreateTreeItemContainerGenerator<TVItem>() where TVItem: TreeViewItem, new()
         {
             return new TreeItemContainerGenerator<TVItem>(
                 this,

src/Avalonia.Controls/TreeViewItem.cs

@@ -96,7 +96,7 @@ namespace Avalonia.Controls
         protected override IItemContainerGenerator CreateItemContainerGenerator() => CreateTreeItemContainerGenerator<TreeViewItem>();
 
         /// <inheritdoc/>
-        protected virtual ITreeItemContainerGenerator CreateTreeItemContainerGenerator<TVItem>()
+        protected ITreeItemContainerGenerator CreateTreeItemContainerGenerator<TVItem>()
             where TVItem: TreeViewItem, new()
         {
             return new TreeItemContainerGenerator<TVItem>(