Alarm: add support for power off alarm

When phone is in Power-Off Alarm boot mode:

    1. At the correct booting place to trigger the Power-Off Alarm UI
    2. At the Power-Off Alarm alert UI, don't dispatch related physical
           key, such as home, search, etc

Change-Id: I60ede3bde21d26eafb0610946f6d8bf884c85ddb