This class implements a service. The service is started by AlarmActivity, which contains a repeating countdown timer that sends a PendingIntent. The user starts and stops the timer with buttons in the UI.

内容：1、程序目的：使用定时器学习 倒计时 红绿灯原理 主要程序与倒计时相同 2、硬件要求：数码管、晶振12M 3、P1口连接至LED用于显示红绿灯(p1接锁存器，八段数码管的位信号和断信号都通过p1口锁存起来，这样可以节省端口) 4、数码管用于显示时间-Contents: 1, the program purpose: to learn to use the countdown timer with countdown traffic light principl