refactor: update sender name retrieval logic in push notifications for improved clarity

Author: diegolmello

android/app/src/main/java/chat/rocket/reactnative/notification/CustomPushNotification.java

@@ -550,7 +550,7 @@ private void notificationStyle(Notification.Builder notification, int notId, Bun
 
                     String displaySenderName = (ejson != null && ejson.senderName != null && !ejson.senderName.isEmpty())
                             ? ejson.senderName
-                            : (ejson != null && ejson.sender != null ? ejson.sender.name : "Unknown");
+                            : (ejson != null && ejson.sender != null ? ejson.sender.username : title);
 
                     if (Build.VERSION.SDK_INT < Build.VERSION_CODES.P) {
                         messageStyle.addMessage(m, timestamp, displaySenderName);