update_engine: Add Update Time Restrictions

Implementation for the update time restrictions policy where the admin
is allowed to choose time interval in which updates will not be checked.

- Adds WeeklyTime and WeeklyTimeInterval classes to update_engine to be
able to easily do interval range checking and time operations easily in
the policy. Added the wiring so the classes can be used as Values and
BoxedValues.
- Adds disallowed_intervals member to device_policy_provider, since this
contains the policy dictated disallowed intervals. The intervals are
obtained from libpolicy, a function was added to convert them to the
new WeeklyTime classes. Added the corresponding changes to the device
policy provider header and interface.
- Added the policy to chromeos_policy.cc so that it's taken into account
in UpdateCheckAllowed.

BUG=chromium:852860
TEST=cros_workon_make update_engine --test

Change-Id: If62a2b3650adf149ba87790345e1eb62f0e1bb1f
Reviewed-on: https://chromium-review.googlesource.com/1119397
Commit-Ready: Adolfo Higueros <adokar@google.com>
Tested-by: Adolfo Higueros <adokar@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/update_manager/device_policy_provider.h b/update_manager/device_policy_provider.h
index 22613a1..c7e9d3a 100644
--- a/update_manager/device_policy_provider.h
+++ b/update_manager/device_policy_provider.h
@@ -27,6 +27,7 @@
 #include "update_engine/update_manager/rollback_prefs.h"
 #include "update_engine/update_manager/shill_provider.h"
 #include "update_engine/update_manager/variable.h"
+#include "update_engine/update_manager/weekly_time.h"
 
 namespace chromeos_update_manager {
 
@@ -75,6 +76,11 @@
 
   virtual Variable<bool>* var_allow_kiosk_app_control_chrome_version() = 0;
 
+  // Variable that contains the time intervals during the week for which update
+  // checks are disallowed.
+  virtual Variable<WeeklyTimeIntervalVector>*
+  var_disallowed_time_intervals() = 0;
+
  protected:
   DevicePolicyProvider() {}