Tabbar position on Android always at the top

Hi all.

### Version
- react-native-router-flux v4.0.5
- react v16.5.
- react-native v0.57.1

I have switched from another Tab component to the Router-Flux one.
Everything is working fine, except from the Tabbar position itself on android.

### Expected behaviour
```

{
console.log('Back to initial and also print this');
}}
swipeEnabled
showLabel={false}
tabBarStyle={styles.tabBarStyle}
activeBackgroundColor="white"
inactiveBackgroundColor="rgba(255, 0, 0, 0.5)"
tabBarPosition="bottom"
>
```
This is the code I have copied from the example, and I have added the ` tabBarPosition="bottom"`.

### Actual behaviour
On iOS the Tabbar is at the bottom, but on Android is at the top.

Maybe I am wrong, but for what I have read on the documentation, the default behaviour on iOS is at the botton and for Android at the top, but I thought that it was possible also on android to place the tabbar at the bottom.

Isn't it?
Thank you in advance.