How to prevent load login page again if already have session after user backhandler exitapp()?

### Version

- react-native-router-flux v4.0.1
- react v16.4.1
- react-native 0.55.4

### Expected behaviour

after tap2x exit apps, and open apps again go to scene home

### Actual behaviour
after tap2x exit apps, and open apps again go to scene login

### Steps to reproduce
```
componentDidMount() {
BackHandler.addEventListener('hardwareBackPress', this.handleBackButton);
}
componentWillUnmount() {
BackHandler.removeEventListener('hardwareBackPress', this.handleBackButton);
}
handleBackButton = () => {
if (this.state.doubleBackToExitPressedOnce) {
BackHandler.exitApp();
}
else if(NavigationActions.currentScene === 'home'){
ToastAndroid.show('Press back again to exit', ToastAndroid.SHORT);
this.setState({ doubleBackToExitPressedOnce: true });
// BackHandler.exitApp();
setTimeout(() => {
this.setState({ doubleBackToExitPressedOnce: false });
}, 2000);
return true;
}

}
```

on router:
```
< Scene initial component={LoginScreen} hideNavBar titleStyle={styles.title} title='' />
< Scene key="home" component={WelcomeScreen} hideNavBar title='' navigationBarStyle={styles.navBar} titleStyle={styles.title} back drawerLockMode='locked-closed' />
```