More fixing of provider resolution.

Found a few more places that were doing casts instead of calling `GetProvider<T>()`.

src/Avalonia.Controls/Automation/Peers/ItemsControlAutomationPeer.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Automation.Peers
                 if (!_searchedForScrollable)
                 {
                     if (Owner.GetValue(ListBox.ScrollProperty) is Control scrollable)
-                        _scroller = GetOrCreate(scrollable) as IScrollProvider;
+                        _scroller = GetOrCreate(scrollable).GetProvider<IScrollProvider>();
                     _searchedForScrollable = true;
                 }
 

src/Avalonia.Controls/Automation/Peers/ListItemAutomationPeer.cs

@@ -22,7 +22,7 @@ namespace Avalonia.Automation.Peers
                 if (Owner.Parent is Control parent)
                 {
                     var parentPeer = GetOrCreate(parent);
-                    return parentPeer as ISelectionProvider;
+                    return parentPeer.GetProvider<ISelectionProvider>();
                 }
 
                 return null;