title: BackHandler does not go back more than 1 screen
body: <p>I have this code on each of my screens. Pressing the android back button goes back 1 screen. Pressing android back button again does not do anything. Expected result would be to keep going back as long as there are more screens in the stack. What's missing?</p>
  
  <pre><code>componentWillMount() {
      BackHandler.addEventListener('hardwareBackPress', () =&gt; {
          this.props.navigation.goBack();
          return true;
      });
  }
  
  componentWillUnmount() {
    BackHandler.removeEventListener('hardwareBackPress')
  }
  </code></pre>