title: Problem using DecorView with shared element transitions
body: <p>I want to use the background of an Activity as a part of <code>SharedElementTransition</code>, but when adding <code>DecorView</code> to the transition, the app crashes with the following trace:</p>
  <pre><code>java.lang.ClassCastException: android.view.ViewRootImpl cannot be cast to android.view.ViewGroup
          at android.app.ActivityTransitionCoordinator.setSharedElementMatrices(ActivityTransitionCoordinator.java:571)
          at android.app.ActivityTransitionCoordinator.moveSharedElementsToOverlay(ActivityTransitionCoordinator.java:858)
          at android.app.EnterTransitionCoordinator.viewsReady(EnterTransitionCoordinator.java:144)
          at android.app.EnterTransitionCoordinator.lambda$triggerViewsReady$0(EnterTransitionCoordinator.java:164)
          at android.app.-$$Lambda$EnterTransitionCoordinator$wYWFlx9zS3bxJYkN44Bpwx_EKis.run(Unknown Source:4)
          at com.android.internal.view.OneShotPreDrawListener.onPreDraw(OneShotPreDrawListener.java:78)
          at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:977)
          at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2478)
          at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1462)
          at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7187)
          at android.view.Choreographer$CallbackRecord.run(Choreographer.java:949)
          at android.view.Choreographer.doCallbacks(Choreographer.java:761)
          at android.view.Choreographer.doFrame(Choreographer.java:696)
          at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:935)
          at android.os.Handler.handleCallback(Handler.java:873)
          at android.os.Handler.dispatchMessage(Handler.java:99)
          at android.os.Looper.loop(Looper.java:193)
          at android.app.ActivityThread.main(ActivityThread.java:6669)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:951)
  
  </code></pre>
  <p>The main reason for the crash is that the parent of <code>DecorView</code> when called by <code>view.getParent()</code> is not a <code>ViewGroup</code>. Is there another way to animate the background of an Activity when entering it from another Activity as a part of <code>SharedElementTransition</code>?</p>
  <p>It's worth mentioning that I'm using <code>changeImageTransform</code> in <code>windowSharedElementEnterTransition</code> right now.</p>