Fix/linux atspi backend (#21072)

* Fix ATSPI SyncContext to UIThread

* Update Avalonia.DBus submodule to fix empty array serialization and error handling

* use public api and revert to instantiation.

external/Avalonia.DBus

@@ -1 +1 @@
-Subproject commit f91a822c258476f185e51112388775591e6ef9d6
+Subproject commit 864a05282841bf04006890f04d11d60d1a046aa9

src/Avalonia.FreeDesktop.AtSpi/AtSpiServer.cs

@@ -61,7 +61,7 @@ namespace Avalonia.FreeDesktop.AtSpi
                 _isEmbedded = false;
             }
 
-            _syncContext = new AvaloniaSynchronizationContext(DispatcherPriority.Normal);
+            _syncContext = new AvaloniaSynchronizationContext(Dispatcher.UIThread, DispatcherPriority.Default);
 
             var address = await GetAccessibilityBusAddressAsync();