react-navigation 去除导航阴影

let platformContainerStyles;
if (Platform.OS === 'ios') {
  platformContainerStyles = {
    borderBottomWidth: StyleSheet.hairlineWidth,
    borderBottomColor: '#A7A7AA',
  };
} else {
  platformContainerStyles = {
    shadowColor: 'black',
    shadowOpacity: 0.1,
    shadowRadius: StyleSheet.hairlineWidth,
    shadowOffset: {
      height: StyleSheet.hairlineWidth,
    },
    elevation: 4,
  };
}