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/boxed_value_unittest.cc b/update_manager/boxed_value_unittest.cc
index 2a086a6..83d8de7 100644
--- a/update_manager/boxed_value_unittest.cc
+++ b/update_manager/boxed_value_unittest.cc
@@ -231,4 +231,14 @@
   EXPECT_EQ("DeleterMarker:true", value.ToString());
 }
 
+TEST(UmBoxedValueTest, UpdateRestrictionsToString) {
+  EXPECT_EQ(
+      "None",
+      BoxedValue(new UpdateRestrictions(UpdateRestrictions::kNone)).ToString());
+  EXPECT_EQ("Flags: RestrictDownloading",
+            BoxedValue(new UpdateRestrictions(
+                           UpdateRestrictions::kRestrictDownloading))
+                .ToString());
+}
+
 }  // namespace chromeos_update_manager