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/MaskedViewExample.js

@@ -22,11 +22,12 @@ const {
   View,
 } = require('react-native');
 
-class MaskedViewExample extends React.Component<{}, $FlowFixMeState> {
-  static title = '<MaskedViewIOS>';
-  static description =
-    'Renders the child view with a mask specified in the `renderMask` prop.';
+type Props = $ReadOnly<{||}>;
+type State = {|
+  alternateChildren: boolean,
+|};
 
+class MaskedViewExample extends React.Component<Props, State> {
   state = {
     alternateChildren: true,
   };
@@ -181,4 +182,14 @@ const styles = StyleSheet.create({
   },
 });
 
-module.exports = MaskedViewExample;
+exports.title = '<MaskedViewIOS>';
+exports.description =
+  'Renders the child view with a mask specified in the `renderMask` prop.';
+exports.examples = [
+  {
+    title: 'Simple masked view',
+    render: function(): React.Element<typeof MaskedViewExample> {
+      return <MaskedViewExample />;
+    },
+  },
+];