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
Exempt-From-Owner-Approval: cherry-pick from nyc-iot-dev

Change-Id: I655adf23cae44c63079bfa9dc18ba8ca65d7a304
(cherry picked from commit e22f2ddfec92623d38efbf17c17917f68e52907a)
diff --git a/update_manager/android_things_policy.h b/update_manager/android_things_policy.h
index 6f8f4ad..b45e955 100644
--- a/update_manager/android_things_policy.h
+++ b/update_manager/android_things_policy.h
@@ -37,7 +37,8 @@
                                 std::string* error,
                                 UpdateCheckParams* result) const override;
 
-  // Always returns |EvalStatus::kSucceeded|
+  // Uses the |UpdateRestrictions| to determine if the download and apply can
+  // occur at this time.
   EvalStatus UpdateCanBeApplied(
       EvaluationContext* ec,
       State* state,