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>