libsnapshot:snapuserd: Terminate daemon after applying OTA.

When all threads are terminated, dm-user handler's are removed
from the list. When the last handler is removed, daemon is
shutdown gracefully.

Bug: 183652708
Test: 1: Apply full OTA and verify daemon is terminated; reapply the OTA
      to verify daemon is restarted again.
      2:  vts_libsnapshot_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ibd41223fc0eba884993a533fcc95661f72805db2