Fix NavigationScenesReducer.

Summary:
Fix a bug in NavigationScenesReducer that prevents scenes from re-rendering.
This happens when jumping the index between routes.

The fix is to add an new property `isActive` to `NavigationScene` to indicate the current active scene.

Reviewed By: ericvicenti

Differential Revision: D3479736

fbshipit-source-id: a71419887acd94ad2fead71596ca46419a88efef

Libraries/NavigationExperimental/NavigationTypeDefinition.js

@@ -40,6 +40,7 @@ export type NavigationLayout = {
 
 export type NavigationScene = {
   index: number,
+  isActive: boolean,
   isStale: boolean,
   key: string,
   route: NavigationRoute,