update_engine: Move DLC metadata ownership to update_engine

Add dbus messages so dlcservice can let update_engine know when a DLC is
installed or uninstalled.

BUG=chromium:912666
TEST=unittests, install and uninstall DLCs on DUT.

Cq-Depend: chromium:2112994,chromium:2113254
Change-Id: I35374504afcdaf96b099e343cabe072fc18f1022
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2113134
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Tested-by: Andrew Lassalle <andrewlassalle@chromium.org>
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index ab776e3..f9a66dc 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -98,5 +98,8 @@
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
            send_member="AttemptInstall"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="SetDlcActiveValue"/>
   </policy>
 </busconfig>