DeskClock: add support for power off alarm

When it is alarm boot, it will trigger power off alarm alert view at
the very beinging of the phone boot phase.

During power off alarm alert view:
  - airplane mode is enabled, will restore later
  - keyguard is dismissed, will restore later
  - user can choose to continue power on the device or not

Change-Id: Id5bce67793cef694712b0591be68ef6882be6693