support shutdown animation from bootanim

- init will set sys.powerctl property and run bootanim.
- Use /oem/media/shutdownanimation.zip or
  /system/media/shutdownanimation.zip for animation file.
  If none of them exist, android animation will be used.
- Disable audio playing for shutdown animation.
- Disable TimeCheckThread for shutdown animation. It
  accesses /data partition and can block umount.

bug: 36526187
Test: many reboots

Change-Id: If02c73cda2015317d88d056cd45201208da49946