fix(PlayerPresence): prevent collisions when swapping grab hand

There was an issue where if the grabbing item was swapped between
hands then the player presence collider would resume colliding with
the grabbed object.

This fix ensures the colliders are only turned back on when the
grabbed object is definitely not being grabbed any more.

Assets/VRTK/Scripts/VRTK_PlayerPresence.cs

@@ -199,15 +199,25 @@ private void OnGrabObject(object sender, ObjectInteractEventArgs e)
         {
             if (e.target)
             {
+                StopCoroutine("RestoreCollisions");
                 IgnoreCollisions(e.target.GetComponentsInChildren<Collider>(), true);
             }
         }
 
         private void OnUngrabObject(object sender, ObjectInteractEventArgs e)
         {
-            if (e.target && e.target.GetComponent<VRTK_InteractableObject>())
+            if (gameObject.activeInHierarchy)
             {
-                IgnoreCollisions(e.target.GetComponentsInChildren<Collider>(), false);
+                StartCoroutine(RestoreCollisions(e.target));
+            }
+        }
+
+        private IEnumerator RestoreCollisions(GameObject obj)
+        {
+            yield return new WaitForEndOfFrame();
+            if (obj && obj.GetComponent<VRTK_InteractableObject>() && !obj.GetComponent<VRTK_InteractableObject>().IsGrabbed())
+            {
+                IgnoreCollisions(obj.GetComponentsInChildren<Collider>(), false);
             }
         }