touch_updater : Fix so chromeos-touch-update job runs just at boot

Previously, the job consisted of a main script that would run to completion
each time the UI restarted. This means the firmware/config check would
happen on logout too. jrbarnette suggested this change to run the check
as a pre-start so it only blocks the UI on the first boot, not subsequent
UI restarts.

Signed-off-by: Benson Leung <bleung@chromium.org>

BUG=chromium:196823
TEST=Cold boot a system (say, Link).
Check /var/log/messages:
Verify that the update check happens properly.
Here is a normal no-update check :
chromeos-touch-firmware-update[6802]-atmel_mxt_tp: Product ID : 130.1
chromeos-touch-firmware-update[6802]-atmel_mxt_tp: Current Firmware: 1.0.170
chromeos-touch-firmware-update[6802]-atmel_mxt_tp: Updater Firmware: 1.0.170
chromeos-touch-firmware-update[6802]-atmel_mxt_tp: Firmware up to date.
chromeos-touch-config-update[6802]-atmel_mxt_tp: Device config checksum : ae968a
chromeos-touch-config-update[6802]-atmel_mxt_tp: New config checksum : ae968a
chromeos-touch-config-update[6802]-atmel_mxt_tp: Config is up-to-date, no need to update
chromeos-touch-firmware-update[6802]-atmel_mxt_ts: Product ID : 162.0
chromeos-touch-firmware-update[6802]-atmel_mxt_ts: Current Firmware: 1.1.170
chromeos-touch-firmware-update[6802]-atmel_mxt_ts: Updater Firmware: 1.1.170
chromeos-touch-firmware-update[6802]-atmel_mxt_ts: Firmware up to date.
chromeos-touch-config-update[6802]-atmel_mxt_ts: Device config checksum : 7d7cf1
chromeos-touch-config-update[6802]-atmel_mxt_ts: New config checksum : 7d7cf1
chromeos-touch-config-update[6802]-atmel_mxt_ts: Config is up-to-date, no need to update

Log in as Guest. Log out.
Check /var/log/messages again. Check that the above check does not appear around
the time of logout.

Change-Id: I3bb876180073783ec471959f155375a7101d05ca
Reviewed-on: https://gerrit.chromium.org/gerrit/45794
Reviewed-by: Benson Leung <bleung@chromium.org>
Tested-by: Benson Leung <bleung@chromium.org>
Commit-Queue: Benson Leung <bleung@chromium.org>