Add type annotations to render method of react components

Summary:
This is part of enabling types-first mode in xplat/js ([context](https://fb.workplace.com/groups/rn.engineering/permalink/2293015867694617/)).

This diff adds `React.Node` as the return type of the `render` method of react components.

Differential Revision: D17137432

fbshipit-source-id: 415e902d87b6be5c26e4a0af3884a43a89c9be78

RNTester/js/examples/ActivityIndicator/ActivityIndicatorExample.js

@@ -42,7 +42,7 @@ class ToggleAnimatingActivityIndicator extends Component<Props, State> {
     }, 2000);
   }
 
-  render() {
+  render(): Node {
     return (
       <ActivityIndicator
         animating={this.state.animating}