fix(fcm): Correctly map the string representation of the Android Config normal priority. (#452)

* Update AndroidConfig.cs

* Empty commit message

* Empty

Author: zSlicker

FirebaseAdmin/FirebaseAdmin/Messaging/AndroidConfig.cs

@@ -107,7 +107,7 @@ private string PriorityString
                         this.Priority = Messaging.Priority.High;
                         return;
                     case "normal":
-                        this.Priority = Messaging.Priority.High;
+                        this.Priority = Messaging.Priority.Normal;
                         return;
                     default:
                         throw new ArgumentException(