Reduce the priority in AvaloniaSynchronizationContext.

`AvaloniaSynchronizationContext` was previously queuing continuations with `Send` priority, which is the maximum. This was causing the problem described in https://github.com/AvaloniaUI/Avalonia/issues/6930#issuecomment-1008888711. Decrease the priority so that continuations don't get called during `Window` construction on macOS.

src/Avalonia.Base/Threading/AvaloniaSynchronizationContext.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Threading
         /// <inheritdoc/>
         public override void Post(SendOrPostCallback d, object? state)
         {
-           Dispatcher.UIThread.Post(() => d(state), DispatcherPriority.Send);
+           Dispatcher.UIThread.Post(() => d(state), DispatcherPriority.Background);
         }
 
         /// <inheritdoc/>