Update FirebaseClient.js

fix "YOUR_API_KEY" check

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

@@ -6,7 +6,7 @@ const API_URL = "https://fcm.googleapis.com/fcm/send";
 class FirebaseClient {
 
   async send(body, type) {
-		if(FirebaseClient.KEY === 'YOUR_API_KEY'){
+		if(FirebaseConstants.KEY === 'YOUR_API_KEY'){
 			Alert.alert('Set your API_KEY in app/FirebaseConstants.js')
 			return;
 		}