add "shutdown critical" to service

- "shutdown critical" prevents killing the service during
  shutdown. And the service will be started if not running.
- Without it, services will be killed by SIGTERM / SIGKILL during shutdown.
- Even services with "shutdown critical" will be killed if shutdown
  times out.
- Removes ueventd and vold from hard coded list. Each service's rc will
  be updated to add "shutdown critical". watchdogd is still kept in the list.

bug: 37626581
Test: reboot and check last kmsg

Change-Id: Ie8cc699d1efbc59b9a2561bdd40fec64aed5a4bb