[Flow] Some updates, mostly from flow-typed (react-navigation#3682)

1. Remove `NavigationComponent` from `NavigationScreenRouteConfig`. The only context `NavigationScreenRouteConfig` is used in is as an intersection with an object, and as such the only relevant portions of `NavigationScreenRouteConfig` are the object parts.
2. Add static `HEIGHT` variable to `Header` type.
3. In `NavigationContainerProps`, make `onNavigationStateChange` property value nullable.

PS: if in the future you guys would prefer that I separate these sort of PRs into their constituent parts, let me know.

Author: Ashoat

flow/react-navigation.js

@@ -296,7 +296,6 @@ declare module 'react-navigation' {
       } & NavigationScreenRouteConfig);
 
   declare export type NavigationScreenRouteConfig =
-    | NavigationComponent
     | {
         screen: NavigationComponent,
       }
@@ -883,7 +882,9 @@ declare module 'react-navigation' {
   };
   declare export var SafeAreaView: React$ComponentType<_SafeAreaViewProps>;
 
-  declare export var Header: React$ComponentType<HeaderProps>;
+  declare export var Header: React$ComponentType<HeaderProps> & {
+    HEIGHT: number,
+  };
 
   declare type _HeaderTitleProps = {
     children: React$Node,