[xabt] Enable nullable in LayoutAttribute.Partial.cs (#12326)

`LayoutAttribute.FromTypeDefinition()` can legitimately return `null` when no layout attribute exists, but its file disabled nullable analysis.

- Enable nullable reference type analysis for the file.
- Mark the factory return type and `SingleOrDefault()` result as nullable.
- Preserve the existing null-handling behavior.

- Fixes #12325

Author: Copilot

src/Xamarin.Android.Build.Tasks/Mono.Android/LayoutAttribute.Partial.cs

@@ -1,4 +1,4 @@
-#nullable disable
+#nullable enable
 
 using System;
 using System.Collections.Generic;
@@ -22,9 +22,9 @@ partial class LayoutAttribute
 		TypeDefinition? type;
 		ICollection<string>? specified;
 
-		public static LayoutAttribute FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
+		public static LayoutAttribute? FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
 		{
-			CustomAttribute attr = type.GetCustomAttributes ("Android.App.LayoutAttribute")
+			CustomAttribute? attr = type.GetCustomAttributes ("Android.App.LayoutAttribute")
 				.SingleOrDefault ();
 			if (attr == null)
 				return null;