Add group and user for arc-data-snapshotd

BUG=b:160387490
TEST=/sbin/start arc-data-snapshotd

Change-Id: I8f8c1469bb053cf847f6d9a399c5151225a1eafb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2292433
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Polina Bondarenko <pbond@chromium.org>
Auto-Submit: Polina Bondarenko <pbond@chromium.org>
Commit-Queue: Polina Bondarenko <pbond@chromium.org>