code: public void sendUnreadMessagesAck(ArrayList<AVIMMessage> messages, String conversationId) {
      if (AVIMOptions.getGlobalOptions().isOnlyPushCount() && null != messages && messages.size() > 0) {
        Long largestTimeStamp = 0L;
        for (AVIMMessage message : messages) {
          if (largestTimeStamp < message.getTimestamp()) {
            largestTimeStamp = message.getTimestamp();
          }
        }
        AVConnectionManager.getInstance().sendPacket(ConversationAckPacket.getConversationAckPacket(getSelfPeerId(),
                conversationId, largestTimeStamp));
      }
    }
docs: 确认客户端已经拉取到未推送到本地的离线消息
  因为没有办法判断哪些消息是离线消息，所以对所有拉取到的消息都发送 ack
  @param messages
  @param conversationId