make shutdown safe for double calls.

- In new shutdown sequence, volume shutdown is requested in two places:
  system_server and init.
- Make VolumeManager.shutdown safe for double calls.
  It crashed before this change.

bug: 36004738
Test: reboot and check last_kmsg for crash / vdc timeout
Change-Id: I296913959b2647b65d66553073f2032545beba57
1 file changed