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/mock_update_attempter.h b/mock_update_attempter.h
index 9634bab..d64dcbe 100644
--- a/mock_update_attempter.h
+++ b/mock_update_attempter.h
@@ -42,6 +42,8 @@
 
   MOCK_METHOD0(ResetStatus, bool(void));
 
+  MOCK_METHOD0(GetCurrentUpdateAttemptFlags, UpdateAttemptFlags(void));
+
   MOCK_METHOD3(CheckForUpdate, void(const std::string& app_version,
                                     const std::string& omaha_url,
                                     bool is_interactive));