title: White Square is displayed instead of my notification icon
body: <p>I'm trying to build my own notification as soon as I receive a payload from my server in my FCM Receiver, I set my notification icon to my app icon which is in PNG Format, however, my notification is displayed with a white square instead and I don't know why..
  Here's my code:</p>
  
  <pre><code> NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context)
                  .setSmallIcon(R.drawable.icon)
                  .setContentTitle("Your chat is going to expire tomorrow!");
  
          // Creates an explicit intent for an Activity in your app
          Intent resultIntent = new Intent(this, main_activity.class);
  
          resultIntent.putExtra("launchedFromNotification",true);
  
          // The stack builder object will contain an artificial back stack for the
          // started Activity.
          // This ensures that navigating backward from the Activity leads out of
          // your app to the Home screen.
          TaskStackBuilder stackBuilder = TaskStackBuilder.create(this);
          // Adds the back stack for the Intent (but not the Intent itself)
          stackBuilder.addParentStack(main_activity.class);
          // Adds the Intent that starts the Activity to the top of the stack
          stackBuilder.addNextIntent(resultIntent);
          PendingIntent resultPendingIntent =
                  stackBuilder.getPendingIntent(
                          0,
                          PendingIntent.FLAG_UPDATE_CURRENT
                  );
  
          mBuilder.setContentIntent(resultPendingIntent);
          NotificationManager mNotificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
          // mNotificationId is a unique integer your app uses to identify the
          mNotificationManager.notify(MESSAGE_NOTIFICATION_ID, mBuilder.build());
  </code></pre>