title: FlatList contentContainerStyle -> justifyContent: 'center' causes issues with scrolling
body: <p>I have a <code>FlatList</code> component where I render x number of <code>TouchableHighlight</code>. I need all components in the <code>FlatList</code> aligned vertically to center.</p>
  
  <p>The problem is that if I put <code>justifyContent: center</code> in <code>contentContainerStyle</code> nothing happens but, if I add <code>flex: 1</code> to <code>contentContainerStyle</code> I get the result I want. It is cool if I doesn't have to make scroll, but when I have a number of components in the <code>FlatList</code> that forces do scroll to see all of it the scroll start in the center of these list and don't let me scroll. </p>
  
  <p>These is my code:</p>
  
  <pre><code>&lt;FlatList
          style = {{flex: 0.7, backgroundColor: 'red'}}
          data = {this.props.actions}
          contentContainerStyle = {{justifyContent:'center',}}
          keyExtractor={(item, index) =&gt; index}
          renderItem = {({item, index})=&gt;{
            return (
              &lt;TouchableHighlight
                style = {{alignItems: 'center', margin: 8, paddingTop: 8, paddingBottom: 8, //flex: 1,
                borderColor: ConstantesString.colorGrisFlojito, backgroundColor: '#d7deeb',
                borderRadius: 10,
              }}
                underlayColor = {ConstantesString.colorAzulTouched}
                onPress = {()=&gt;{
                  item.action();
                }}&gt;
                &lt;Text
                  style = {{color: '#005288' , textAlign: 'center', fontSize: 20}}
                &gt;
                  {item.name}
                &lt;/Text&gt;
              &lt;/TouchableHighlight&gt;
            )
          }}
        /&gt;
  </code></pre>
  
  <p>I don't know if this is a bug or I am just doing bad.</p>