ptp: Add user and group for Precision Time Protocol services

BUG=b:144279331
TEST=profiles/base/accounts/display-accts.py --lint

Change-Id: I050fa3bf3e66288c91e12d553b71122c71230f1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2253141
Reviewed-by: Allen Webb <allenwebb@google.com>
Tested-by: Pablo Ceballos <pceballos@chromium.org>
Commit-Queue: Pablo Ceballos <pceballos@chromium.org>