title: React Native Animated Listener not triggered when offset is updated
body: <p>At the moment I have an animation that updates an offset and then flattens it at the end to reset for the next animation. I am listening for updates to this animation to coordinate with another but it's not being triggered when the offset is updated.</p>
  
  <pre><code>imagePosition.addListener(this._animateOther)
  </code></pre>
  
  <p>I would expect _animateOther to be triggered as my object moves.</p>
  
  <p>However, this does not trigger _animateOther.</p>
  
  <pre><code>this.state.imagePosition.setOffset( update )
  </code></pre>
  
  <p>Only when the offset is merged back into the value does it trigger _animateOther.</p>
  
  <pre><code>this.state.imagePosition.flattenOffset()
  </code></pre>
  
  <p>Is this a bug and is there any way to listen to offset updates?</p>