AlarmManager alarmManager = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
alarmManager.setInexactRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime(), 15 * 1000 * 60, pendingIntent);
设置不精确(精确的耗电) 设置从开机到现在时间计算(rtc耗电) 代表系统开机开始到现在已过去的时间