Prevent unmount when leaving scene inside lightbox

### Version
- react-native-router-flux v4.2.0
- react v16.12.0
- react-native v0.61.5

### Expected behaviour
If I use Lightbox and navigate outside the root scene, to Scene A, then navigate to Scene B, Scene A does not unmount and pop()ing from Scene B, Scene A does not refresh.

### Actual behaviour
Scene A unmounts and refreshes. If I use Modal instead of Lightbox everything is fine but I need the transparency.

### Steps to reproduce
1. you are in one of the Scenes inside the root Scene.
2. you navigate to one of the Scenes (A) outside the root Scene, but inside the Lightbox.
3. you navigate to another Scene (B) outside the root Scene, but inside the Lightbox. Scene A unmounts (which I like to prevent)
4. pop()
5. Scene A reloads (instead of just being there like if I used Modal instead of Lightbox)

### Reproducible Demo
```









...
Actions.a();
...
Actions.b();
...
Actions.pop();
```