Summary:
With nested NavigatorIOS components:
```
<NavigatorIOS initialRoute={{
component: ComponentWithAnotherNavigatorIOSInSubtree
title: 'xyz'
}}>
```
Navigating (via push etc.) in ComponentWithAnotherNavigatorIOSInSubtree's `navigator` caused an invariant error, making apps with this structure unusable. The reason was that the `navigationComplete` nativeEvent was being propagated to the outer Navigator due to React's automatic event bubbling, making the outer NavigatorIOS incorrectly think it was holding navigation stack state inconsistent with its native UINavigationController.
Concretely, if the outer navigation stack is empty except for its initial state and something is pushed onto the inner stack, the outer NavigatorIOS suddenly complains that it has 2 items on its stack rather than 1. In reality, the outer Navigator still only has 1 item on its stack but the inner Navigator's event (containing information about the inner Navigator) incorrectly bubbles up and reaches the outer Navigator too.
Closes https://github.com/facebook/react-native/pull/9828
Differential Revision: D4030167
Pulled By: ericvicenti
fbshipit-source-id: d04de3d5b70b4862a78610c92701ebdab2b047dd
73c5360470