Explicitly set IsEnabled to false when the SU package is not present

Co-Authored-By: Kurtis <[email protected]>

Author: MaxWang-MS

Assets/MRTK/Providers/Experimental/WindowsSceneUnderstanding/WindowsSceneUnderstandingObserver.cs

@@ -158,6 +158,8 @@ public override void Enable()
             cancelToken = cancelTokenSource.Token;
 
             task = Task.Run(() => RunObserverAsync(cancelToken));
+#else
+            IsEnabled = false;
 #endif // SCENE_UNDERSTANDING_PRESENT
         }