installer: Add systemd service file for crx-import, install-completed

Added the systemd service files.
BUG=chromium:583671
CQ-DEPEND=CL:354960
TEST=Created /mnt/stateful_partition/.install_completed file,
  rebooted and noticed that the commands are successfully executed
  for install-completed. Checked that both services work with
  "systemctl status" and "initctl status".

Change-Id: I125c795ba833137c9467aa5d9ed848df881a4cf8
Reviewed-on: https://chromium-review.googlesource.com/354950
Commit-Ready: Sabin Floares <sabin.floares@intel.com>
Tested-by: Sabin Floares <sabin.floares@intel.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed