update_engine: Don't require reboot after install.

We don't require a reboot after installing a DLC. Instead just set
Update Engine's status to idle.

Resubmitting now that crbug.com/905704 is fixed.

BUG=chromium:907205,chromium:907632
TEST=Attempt a DLC install, UE no longer prompts for a reboot.

(cherry picked from commit d772aaeb)

Change-Id: I061ee8d6268a922ae21f310211acea5b645a3ebb
Reviewed-on: https://chromium-review.googlesource.com/1362572


Commit-Ready: Colin Howes <chowes@google.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>