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_engine.gyp b/update_engine.gyp
index 30daf2a..b09a401 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -294,6 +294,8 @@
         'update_manager/real_updater_provider.cc',
         'update_manager/state_factory.cc',
         'update_manager/update_manager.cc',
+        'update_manager/update_time_restrictions_policy_impl.cc',
+        'update_manager/weekly_time.cc',
         'update_status_utils.cc',
       ],
       'conditions': [
@@ -579,7 +581,9 @@
             'update_manager/real_updater_provider_unittest.cc',
             'update_manager/umtest_utils.cc',
             'update_manager/update_manager_unittest.cc',
+            'update_manager/update_time_restrictions_policy_impl_unittest.cc',
             'update_manager/variable_unittest.cc',
+            'update_manager/weekly_time_unittest.cc',
           ],
         },
       ],