Refactor <NavigationHeader /> API.

Summary:- All the public sub component renderers should implement the interface
  NavigationSceneRenderer, which will help to reuse renderer or
  replace renders for different composition.

- Perf improvement. <NavigationHeader /> is rendering way more
  sub component than necessary, we shall fix that.

- No UI or behavior change.

Reviewed By: ericvicenti

Differential Revision: D3091442

fb-gh-sync-id: fba5f7ce74597fa6036b5b216c02b06052801983
shipit-source-id: fba5f7ce74597fa6036b5b216c02b06052801983

Libraries/NavigationExperimental/NavigationTypeDefinition.js

@@ -107,3 +107,8 @@ export type NavigationReducer = (
 export type NavigationSceneRenderer = (
   props: NavigationSceneRendererProps,
 ) => ?ReactElement;
+
+export type NavigationStyleInterpolator = (
+  props: NavigationSceneRendererProps,
+) => Object;
+