Revert "update_engine: Don't require reboot after install."

This reverts commit d772aaeb.

Reason for revert: crbug.com/905704

Original change's description:
> 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.
>
> BUG=chromium:907205,chromium:907632
> TEST=Attempt a DLC install, UE no longer prompts for a reboot.
>
> Change-Id: I4fba804ba8b62b8e781279c988aba0e1f21fe3cf
> Reviewed-on: https://chromium-review.googlesource.com/1344916


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

TBR=norvez@chromium.org,ahassani@chromium.org,xiaochu@chromium.org,chromiumos-cl-exonerator@appspot.gserviceaccount.com,chowes@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:907205, chromium:907632
Change-Id: I4c95c7c0acacfb2a6589a7b2b82d445a8ef1f7d0
Reviewed-on: https://chromium-review.googlesource.com/1359108


Commit-Ready: Xiaochu Liu <xiaochu@chromium.org>
Tested-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>