Adding AppNotificationConferencingConfig to AppxManifest.xml (#5026)

Author: satkh

build/NuSpecs/AppxManifest.xml

@@ -64,6 +64,7 @@
         <ActivatableClass ActivatableClassId="Microsoft.Windows.AppNotifications.AppNotification" ThreadingModel="both" />
         <ActivatableClass ActivatableClassId="Microsoft.Windows.AppNotifications.AppNotificationProgressData" ThreadingModel="both" />
         <ActivatableClass ActivatableClassId="Microsoft.Windows.AppNotifications.AppNotificationActivatedEventArgs" ThreadingModel="both" />
+        <ActivatableClass ActivatableClassId="Microsoft.Windows.AppNotifications.AppNotificationConferencingConfig" ThreadingModel="both" />
 
         <!-- AppNotificationBuilder -->
         <ActivatableClass ActivatableClassId="Microsoft.Windows.AppNotifications.Builder.AppNotificationBuilder" ThreadingModel="both" />