update_engine: Refresh device policy when it is updated.

The session manager emits a signal whenever the device policy is
updated. This patch makes update_engine listen for that signal and
reload the device policy.

BUG=chromium:423037
TEST=deployed update_engine and checked the policy is refreshed.

Change-Id: I71dd7047e53d49f3402e1f9f4a67ec8cbd3739d3
Reviewed-on: https://chromium-review.googlesource.com/235884
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
8 files changed