Do not update detected time on every new version downloaded

Currently in Chrome OS the upgrade_detected_time is reset
on each new update downloaded. It causes RelaunchNotificationPeriod policy
could never force install new version of Chrome OS.
upgrade_detected_time should be set once when first upgrade downloaded and
becomes null due ChromeOS relaunch to install new upgrade.

Bug: 928266
Change-Id: Ibd7c8829685e278e5579c08a11e70cc848bec426
Reviewed-on: https://chromium-review.googlesource.com/c/1452168
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Roman Aleksandrov <raleksandrov@google.com>
Cr-Commit-Position: refs/heads/master@{#629574}
4 files changed