Add flag for restricting downloads

This adds a flag that can be used to turn off the ability to download
and apply updates from the API.  This value applies to all future
update checks that the update_engine performs.  Changing this value
during an update check has no effect on the current update check.

Bug: 66016687
Test: unit-tests, manual OTA

Change-Id: I655adf23cae44c63079bfa9dc18ba8ca65d7a304
(cherry picked from commit e22f2ddfec92623d38efbf17c17917f68e52907a)
diff --git a/update_manager/fake_updater_provider.h b/update_manager/fake_updater_provider.h
index a7c15b5..3e03d43 100644
--- a/update_manager/fake_updater_provider.h
+++ b/update_manager/fake_updater_provider.h
@@ -85,6 +85,10 @@
     return &var_forced_update_requested_;
   }
 
+  FakeVariable<UpdateRestrictions>* var_update_restrictions() override {
+    return &var_update_restrictions_;
+  }
+
  private:
   FakeVariable<base::Time>
       var_updater_started_time_{  // NOLINT(whitespace/braces)
@@ -121,6 +125,10 @@
   FakeVariable<UpdateRequestStatus>
       var_forced_update_requested_{  // NOLINT(whitespace/braces)
     "forced_update_requested", kVariableModeAsync};
+  FakeVariable<UpdateRestrictions> var_update_restrictions_{
+      // NOLINT(whitespace/braces)
+      "update_restrictions",
+      kVariableModePoll};
 
   DISALLOW_COPY_AND_ASSIGN(FakeUpdaterProvider);
 };