Export getNavigation as public API, test redux (#4489)

This unblocks the fixing of react-navigation-redux-helpers to unbreak redux support in 2.3

https://github.com/react-navigation/react-navigation-redux-helpers/pull/37

All redux users will need to change their code to reflect the changes made here to `ReduxExample/AppNavigator`

packages/react-navigation/examples/ReduxExample/src/navigators/AppNavigator.js

@@ -31,7 +31,6 @@ class AppWithNavigationState extends React.Component {
       dispatch,
       nav,
       AppNavigator.router,
-      () => this.props.screenProps,
       () => this._navigation
     );
     return <AppNavigator navigation={this._navigation} />;