[Core] Avoid ambiguous call

CS0121: The call is ambiguous between the following methods or
properties: 'Device.InvokeOnMainThreadAsync(Action)' and
'Device.InvokeOnMainThreadAsync(Func<Task>)'

dotnet/roslyn#14885
dotnet/csharplang#98

Xamarin.Forms.Core/Device.cs

@@ -124,7 +124,7 @@ public static Task<T> InvokeOnMainThreadAsync<T>(Func<T> func)
 
 		public static Task InvokeOnMainThreadAsync(Action action)
 		{
-			object wrapAction() { action(); return null; }
+			string wrapAction() { action(); return null; }
 			return InvokeOnMainThreadAsync(wrapAction);
 		}