以下是一个在Android中实现推送消息在几秒后消失的代码示例：

1. 创建一个新的类NotificationUtils：

import android.app.Notification;
import android.app.NotificationManager;
import android.content.Context;
import android.os.Handler;
import android.os.Looper;
import android.support.v4.app.NotificationCompat;

public class NotificationUtils {
    private static final int NOTIFICATION_ID = 1;
    private static final int DELAY_MILLIS = 3000; // 3秒

    public static void showNotification(Context context, String title, String message) {
        NotificationCompat.Builder builder = new NotificationCompat.Builder(context, "channel_id")
                .setSmallIcon(R.drawable.ic_notification)
                .setContentTitle(title)
                .setContentText(message)
                .setAutoCancel(true);

        Notification notification = builder.build();

        NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.notify(NOTIFICATION_ID, notification);

        // 注册一个延迟的任务，在指定时间后取消通知
        new Handler(Looper.getMainLooper()).postDelayed(new Runnable() {
            @Override
            public void run() {
                notificationManager.cancel(NOTIFICATION_ID);
            }
        }, DELAY_MILLIS);
    }
}

2. 在你的推送消息接收到的地方调用NotificationUtils.showNotification()方法来显示通知，例如在onMessageReceived()方法中：

@Override
public void onMessageReceived(RemoteMessage remoteMessage) {
    // 解析推送消息
    String title = remoteMessage.getData().get("title");
    String message = remoteMessage.getData().get("message");

    // 显示通知
    NotificationUtils.showNotification(getApplicationContext(), title, message);
}

请确保你已经创建了一个合适的通知渠道（channel_id）并在AndroidManifest.xml文件中声明了相关权限和服务。