Remove Unity 2018 support for scene understanding

Author: MaxWang-MS

Assets/MRTK/Providers/Experimental/WindowsSceneUnderstanding/Editor/MRTK.WSU.Editor.asmdef

@@ -15,5 +15,9 @@
     "overrideReferences": false,
     "precompiledReferences": [],
     "autoReferenced": true,
-    "defineConstraints": []
+    "defineConstraints": [
+        "UNITY_2019_4_OR_NEWER"
+    ],
+    "versionDefines": [],
+    "noEngineReferences": false
 }

Assets/MRTK/Providers/Experimental/WindowsSceneUnderstanding/Editor/WindowsSceneUnderstandingConfigChecker.cs

@@ -17,7 +17,9 @@
     "overrideReferences": false,
     "precompiledReferences": [],
     "autoReferenced": true,
-    "defineConstraints": [],
+    "defineConstraints": [
+        "UNITY_2019_4_OR_NEWER"
+    ],
     "versionDefines": [
         {
             "name": "com.microsoft.mixedreality.sceneunderstanding",

Assets/MRTK/Providers/Experimental/WindowsSceneUnderstanding/Editor/WindowsSceneUnderstandingConfigChecker.cs.meta

@@ -113,13 +113,13 @@ public override void Reset()
         /// <inheritdoc />
         public override void Initialize()
         {
-            base.Initialize();
 #if !SCENE_UNDERSTANDING_PRESENT
             if (Application.isPlaying)
             {
                 Debug.LogWarning("The required package Microsoft.MixedReality.SceneUnderstanding is not installed or properly configured. Please visit https://docs.microsoft.com/windows/mixed-reality/mrtk-unity/features/spatial-awareness/scene-understanding for more information.");
             }
 #else
+            base.Initialize();
             sceneEventData = new MixedRealitySpatialAwarenessEventData<SpatialAwarenessSceneObject>(EventSystem.current);
             CreateQuadFromExtents(normalizedQuadMesh, 1, 1);
 
@@ -142,17 +142,20 @@ public override void Initialize()
 #endif // SCENE_UNDERSTANDING_PRESENT
         }
 
-#if SCENE_UNDERSTANDING_PRESENT
-
         /// <inheritdoc />
         public override void Enable()
         {
+#if SCENE_UNDERSTANDING_PRESENT
+            base.Enable();
             // Terminate the background thread when we stop in editor.
             cancelToken = cancelTokenSource.Token;
 
             task = Task.Run(() => RunObserverAsync(cancelToken));
+#endif // SCENE_UNDERSTANDING_PRESENT
         }
 
+#if SCENE_UNDERSTANDING_PRESENT
+
         /// <inheritdoc />
         public override void Update()
         {