Add flow types RNTester examples (#22829)

Summary:
This PR adds flow types for the RNTester examples, and updates all of the RNTester examples to match the flow type consistently.

Previously, there was a mix of static class definitions and whether or not pages exported examples or a component. Now we will always export the same way, enforced by flow types

Note: I also fixed most of the $FlowFixMe in changed components
Pull Request resolved: https://github.com/facebook/react-native/pull/22829

Reviewed By: cpojer

Differential Revision: D13563191

Pulled By: rickhanlonii

fbshipit-source-id: b697e3346a863d1b130881592b0522a96c202b63

RNTester/js/createExamplePage.js

@@ -13,12 +13,11 @@
 const React = require('react');
 
 const RNTesterExampleContainer = require('./RNTesterExampleContainer');
-
-import type {ExampleModule} from 'ExampleTypes';
+import type {RNTesterExample} from 'RNTesterTypes';
 
 const createExamplePage = function(
   title: ?string,
-  exampleModule: ExampleModule,
+  exampleModule: RNTesterExample,
 ): React.ComponentType<any> {
   class ExamplePage extends React.Component<{}> {
     render() {