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/binder_bindings/android/brillo/IUpdateEngine.aidl b/binder_bindings/android/brillo/IUpdateEngine.aidl
index b1a1b4f..c85c0dc 100644
--- a/binder_bindings/android/brillo/IUpdateEngine.aidl
+++ b/binder_bindings/android/brillo/IUpdateEngine.aidl
@@ -20,6 +20,7 @@
 import android.brillo.ParcelableUpdateEngineStatus;
 
 interface IUpdateEngine {
+  void SetUpdateAttemptFlags(in int flags);
   void AttemptUpdate(in String app_version, in String omaha_url, in int flags);
   void AttemptRollback(in boolean powerwash);
   boolean CanRollback();