notify(String tag, int id, Notification notification)
Post a notification to be shown in the status bar. If a notification with the same tag and id has already been posted by your application and has not yet been canceled, it will be replaced by the updated information.
notification을 notify 할때 tag값과 id값을 동일하게 주면 새로 뜨지 않고
기존에 떠있던 tag와 id가 일치하는 항목에 덮어서 뜹니다.
