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/Android.mk b/Android.mk
index 2470865..8f1de53 100644
--- a/Android.mk
+++ b/Android.mk
@@ -306,6 +306,7 @@
     real_system_state.cc \
     update_attempter.cc \
     update_manager/android_things_policy.cc \
+    update_manager/api_restricted_downloads_policy_impl.cc \
     update_manager/boxed_value.cc \
     update_manager/default_policy.cc \
     update_manager/enough_slots_ab_updates_policy_impl.cc \