PM: Add an update engine provider.

The UpdaterProvider exports variables for querying the status of an
update process and related settings. Includes a concrete implementation
(RealUpdaterProvider), which currently links directly with update engine
code and pulls information through the SystemState object.  Also
included is a fake implementation (FakeUpdaterProvider) for testing
purposes.

BUG=chromium:346914
TEST=Unit tests.

Change-Id: I6ed5b40f21e43537e78aebf4217d811e149f745b
Reviewed-on: https://chromium-review.googlesource.com/192232
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/SConstruct b/SConstruct
index 2f40697..3703b3e 100644
--- a/SConstruct
+++ b/SConstruct
@@ -241,6 +241,7 @@
                    policy_manager/real_shill_provider.cc
                    policy_manager/real_system_provider.cc
                    policy_manager/real_time_provider.cc
+                   policy_manager/real_updater_provider.cc
                    policy_manager/state_factory.cc
                    postinstall_runner_action.cc
                    prefs.cc
@@ -301,6 +302,7 @@
                             policy_manager/real_shill_provider_unittest.cc
                             policy_manager/real_system_provider_unittest.cc
                             policy_manager/real_time_provider_unittest.cc
+                            policy_manager/real_updater_provider_unittest.cc
                             policy_manager/variable_unittest.cc
                             postinstall_runner_action_unittest.cc
                             prefs_unittest.cc