docs(AppState.js): update example to ES6

Summary:
If this is the way to go, I'll update the rest of the document :)

> Explain the **motivation** for making this change. What existing problem does the pull request solve?

- Update basic usage to latest ES6-7 syntax
- Provide a working simple example of using AppState
Closes https://github.com/facebook/react-native/pull/11879

Differential Revision: D4443891

Pulled By: hramos

fbshipit-source-id: 87433e994ee56050e24a3853f24a94b54f5586d4

Libraries/AppState/AppState.js

@@ -44,25 +44,37 @@ const invariant = require('fbjs/lib/invariant');
  * while `AppState` retrieves it over the bridge.
  *
  * ```
- * getInitialState: function() {
+ * import React, {Component} from 'react'
- *   return {
+ * import {AppState, Text} from 'react-native'
- *     currentAppState: AppState.currentState,
+ *
- *   };
+ * class AppStateExample extends Component {
- * },
+ *
- * componentDidMount: function() {
+ *   state = {
+ *     appState: AppState.currentState
+ *   }
+ *
+ *   componentDidMount() {
  *     AppState.addEventListener('change', this._handleAppStateChange);
- * },
+ *   }
- * componentWillUnmount: function() {
+ *
+ *   componentWillUnmount() {
  *     AppState.removeEventListener('change', this._handleAppStateChange);
- * },
+ *   }
- * _handleAppStateChange: function(currentAppState) {
+ *
- *   this.setState({ currentAppState, });
+ *   _handleAppStateChange = (nextAppState) => {
- * },
+ *     if (this.state.appState.match(/inactive|background/) && nextAppState === 'active') {
- * render: function() {
+ *       console.log('App has come to the foreground!')
+ *     }
+ *     this.setState({appState: nextAppState});
+ *   }
+ *
+ *   render() {
  *     return (
- *     <Text>Current state is: {this.state.currentAppState}</Text>
+ *       <Text>Current state is: {this.state.appState}</Text>
  *     );
- * },
+ *   }
+ *
+ * }
  * ```
  *
  * This example will only ever appear to say "Current state is: active" because