添加常驻Notification

　　private static final int NOTIFICATION_ID=250;  //用来标示notification，通过notificatinomanager来发布同样标示的notification将更新旧的通知，同时取消notification也需要这个标示（android很多地方都用到标示这么一个玩意）
　　
    private void setNotification(String note) {
　　　　　//获取系统通知服务
        NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        Notification notification = new Notification();
        notification.icon = R.drawable.notify;   //通知图标，官方文档中说必须有icon才能显示出notification
        notification.when = System.currentTimeMillis();
        notification.tickerText = note;    //通知出现在标题栏时显示的内容
        notification.flags = Notification.FLAG_ONGOING_EVENT; // 设置常驻Flag
        notification.defaults = Notification.DEFAULT_SOUND;   //设置notification的提示音为系统默认提示声音
        notification.setLatestEventInfo(this, "提示消息", note, null);
        notificationManager.notify(NOTIFICATION_ID, notification);
    }

    // 取消通知
    private void cancelNotification() {
        NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        notificationManager.cancel(NOTIFICATION_ID);
    }