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>
diff --git a/update_manager/state_factory.cc b/update_manager/state_factory.cc
index 745f0f5..54c0205 100644
--- a/update_manager/state_factory.cc
+++ b/update_manager/state_factory.cc
@@ -29,7 +29,7 @@
   unique_ptr<RealConfigProvider> config_provider(
       new RealConfigProvider(system_state->hardware()));
   unique_ptr<RealDevicePolicyProvider> device_policy_provider(
-      new RealDevicePolicyProvider(policy_provider));
+      new RealDevicePolicyProvider(dbus, policy_provider));
   unique_ptr<RealRandomProvider> random_provider(new RealRandomProvider());
   unique_ptr<RealShillProvider> shill_provider(
       new RealShillProvider(dbus, clock));