Add covariance back to IElementConfiguration (#5771)

* add covariance back

* - remove confusing comment

Xamarin.Forms.Core/IElementConfiguration.cs

@@ -1,8 +1,7 @@
 
 namespace Xamarin.Forms
 {
-	// Don't make this generic covariant as it causes UWP performance to tank
-	public interface IElementConfiguration<TElement> where TElement : Element
+	public interface IElementConfiguration<out TElement> where TElement : Element
 	{
 		IPlatformElementConfiguration<T, TElement> On<T>() where T : IConfigPlatform;
 	}