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/binder_service_brillo.h b/binder_service_brillo.h
index 51f0fab..81a4e4c 100644
--- a/binder_service_brillo.h
+++ b/binder_service_brillo.h
@@ -51,6 +51,7 @@
   void SendPayloadApplicationComplete(ErrorCode error_code) override {}
 
   // android::brillo::BnUpdateEngine overrides.
+  android::binder::Status SetUpdateAttemptFlags(int flags) override;
   android::binder::Status AttemptUpdate(const android::String16& app_version,
                                         const android::String16& omaha_url,
                                         int flags) override;