title: Cannot set disabled opacity on TouchableOpacity
body: <p>I am trying to add functionality to disable functionality to a button. I want the button to be non-clickable and have lower opacity. When I click the button, I set the state in my outer component that the button should be disabled (this is while waiting for response from a server). The non-clickable part works, but I cannot get it to lower its opacity after it has been clicked.</p>
  
  <p><strong>Component</strong></p>
  
  <pre><code>&lt;MyButton onPress={ this.setWaiting.bind(this) }
            disabled={ this.state.waiting } /&gt;
  </code></pre>
  
  <p><strong>MyButton</strong></p>
  
  <pre><code>const disabledStyle = {
      opacity: 0.2,
  }
  
  ....
  
  &lt;TouchableOpacity
      style={ [styles.button, props.disabled &amp;&amp; disabledStyle] }
      onPress={ onPress }
      disabled={ props.disabled }&gt;
          ....
  &lt;/TouchableOpacity&gt;
  </code></pre>
  
  <p>If I change <code>disabledStyle</code> to set another background color though, it works as expected and changes the background color while <code>disabled</code> is true.</p>