do not sleep if it is shutting down

do not sleep if it is shutting down

- Various sleep(5) for vold shutdown can increase shutdown time a lot.
- If it is shutting down, do not sleep at all. init will take care of
  active partitions if not unmounted.

bug: 64143519
Test: reboot and check logs from vold, check if "ShutdownThread: Shutdown wait timed out" happens.
Change-Id: I7cb91427ad2205fe23a054d255caf7ffdfd9f6c3