Add support for power off alarm

1. Only install power off alarm apps;
Power off alarm should be least affected when it is firing. There
should be no other applications with notifications and sounds(eg.
phone application and message application). So when it is alarm
boot, we should only install necessary applications for power off
alarm and system's boot-up.

We need to ensure that power off alarm works on both normal status
and encrypt status.

2. Use file in persist folder to save alarm status and instance time;
We need to distinguish power off alarm from other applications which
use RTC_POWEROFF_WAKEUP alarm type. And it should also work when
vold.decrypt property is trigger_restart_min_framework or "1". As
in this state, android system only has temporary data partition.

File in persist folder can be used in any status.

Change-Id: I7f85a2a7e1d06b69ddb74f376435518314105500