fix(SDKManager): SDK Setup unloading

Whenever the SDK Manager was disabled it unloaded the currently
used SDK Setup. This is problematic when the current scene is
changed. This fix makes sure to only unload the SDK Setup in
case the SDK Manager is not set to persist on load.

Assets/VRTK/Scripts/Utilities/SDK/VRTK_SDKManager.cs

@@ -572,7 +572,10 @@ private void OnEnable()
 
         private void OnDisable()
         {
-            UnloadSDKSetup();
+            if (_instance == this && !persistOnLoad)
+            {
+                UnloadSDKSetup();
+            }
         }
 
         private void CreateInstance()