Fixed Platform not being imported and added a base FirebaseConstants file (#346)

Examples/simple-fcm-client/app/FirebaseConstants.js

+
+const FirebaseConstants = {
+	KEY: "YOUR_API_KEY"
+}
+
+export default FirebaseConstants;

Examples/simple-fcm-client/app/PushController.js

 import React, { Component } from "react";
 
+import { Platform } from 'react-native';
+
 import FCM, {FCMEvent, RemoteNotificationResult, WillPresentNotificationResult, NotificationType} from "react-native-fcm";
 
 import firebaseClient from  "./FirebaseClient";