IPHONE X, XR there is space between tabbar and bottom margin.

### Version
Tell us which versions you are using:

- react-native-router-flux v4.0.1
- react v16.6.3
- react-native v0.57.8

### Expected behaviour
Tab bar on iphone X should be on the bottom with no space on top.

### Actual behaviour
But In our case we have white space between tabbar and bottom margin.

### Steps to reproduce
For non-obvious bugs, please fork this component, modify Example project to reproduce your issue and include link here.
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### Reproducible Demo

Please provide a minimized reproducible demonstration of the problem you're reporting.

Issues that come with minimal repro's are resolved much more quickly than issues where a maintainer has to reproduce themselves.