Instructions for Server Key

Instructions currently stop at step `8` which instructs the users to copy and save the server key. However, server key can only be retrieved if `Cloud Messaging API (Legacy)` is enabled, as per instructions for [Legacy SDK](https://learn.microsoft.com/en-us/azure/notification-hubs/notification-hubs-android-push-notification-google-fcm-get-started).  This change takes the instructions from the Legacy SDK and injects them here as well.

Author: samirbanjanovic

articles/notification-hubs/android-sdk.md

@@ -82,6 +82,16 @@ The first step is to create a project in Android Studio:
 
 8. Copy and save the **Server key** for later use. You use this value to configure your hub.
 
+9. If you do not see a **Server key** on the firebase Cloud Messaging tab, follow these additional steps.
+    1. Click on the three-dots menu of the "Cloud Messaging API (Legacy) 🚫 Disabled" Heading
+    2. Follow the offered link to "Manage API in Google Cloud Console".
+    3. In google cloud console, press the button to enable the googlecloudmessaging API.
+    3. Wait a few minutes.
+    4. Go back to your firebase console project Cloud Messaging tab, and refresh the page.
+    5. See that the Cloud Messaging API header has changed to "Cloud Messaging API (Legacy) ✅ Enabled" and now shows a Server Key.
+    
+    ![Screenshot showing the Enable Cloud Messaging API (Legacy) tab.](./media/notification-hubs-enable-firebase-cloud-messaging/notification-hubs-enable-firebase-cloud-messaging-legacy-api.png)
+    
 ## Configure a notification hub
 
 1. Sign in to the [Azure portal](https://portal.azure.com/).