android notif [nfc]: Add TODO for apparent bug in clearing notifications

If the user reads the last messages in several conversations at once,
and those are their only active notifications -- for example, if they
have a couple of recent PMs in different threads, and visit the
"Private messages" view in the web app -- then it looks from the code
here like we'll delete those individual conversations' notifications,
but not the summary for the group, leaving behind a ghost header.

I noticed this just now from reading the code.  I haven't confirmed
empirically that this indeed happens.  If there's a subtle reason
it doesn't, though, we should resolve this TODO by making that clear.

[Added later: Indeed, confirmed this empirically; filed as zulip#5119.]

android/app/src/main/java/com/zulipmobile/notifications/FCMPushNotifications.kt

@@ -143,6 +143,8 @@ private fun removeNotification(context: Context, fcmMessage: RemoveFcmMessage) {
         // counter will be 2 only when summary notification and last notification are
         // present; in this case, we remove summary notification.
         NotificationManagerCompat.from(context).cancel(groupKey, NOTIFICATION_ID)
+        // TODO: What if several conversations get read at once?  Does this leave a ghost group?
+        //   (Yep, it does: #5119.)
     }
 }