Start an animation from a task running on a background thread

In my MainActivity I have a callback whose runs asynchronously on a background thread. I would like to show an Animation (rotation) over an Imageview while the thread is running , so my callback is:

        pull.addChangeListener(new Replication.ChangeListener() {
            @Override
            public void changed(Replication.ChangeEvent event) {

               if (REPLICATION_ACTIVE)) {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                           startAnimation();//I call here to the animation                              
                } else {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                         stopAnimation()
                        }
                    });
                }

The startAnimation() method

 private void startAnimation(){
                isAnimationDone = true;
                imgSincronizacion.setImageResource(R.mipmap.btn_sync_on);
                Animation rotation= AnimationUtils.loadAnimation(getApplicationContext(), R.anim.rotation);
                rotacion.setRepeatMode(Animation.ABSOLUTE);
                rotacion.setRepeatCount(Animation.INFINITE);
                imgSincronizacion.startAnimation(rotation);
        }

Im not facing errors but the animation does not not work. Any idea about how I can animate an ImageView from a background Thread ?