D3321403 [NavigationExperimental][CleanUp]: Rename scene.navigationState to scene.route.

Summary:
[public / experimental API breaking change]

The data type of  `scene.navigationState` is `NavigationRoute`. Rename  `scene.navigationState` to
`scene.route` to avoid confusion such as treating `scene.navigationState` as the actual global navigation
state (type: NavigationState).

Reviewed By: ericvicenti

Differential Revision: D3331076

fbshipit-source-id: 3ed989cc8492d398cbeb1b12186459deb261d1fb

Examples/UIExplorer/NavigationExperimental/NavigationCompositionExample.js

@@ -192,7 +192,7 @@ class ExampleTabScreen extends React.Component {
   _renderTitleComponent(props: NavigationSceneRendererProps) {
     return (
       <NavigationHeader.Title>
-        {stateTypeTitleMap(props.scene.navigationState)}
+        {stateTypeTitleMap(props.scene.route)}
       </NavigationHeader.Title>
     );
   }