Set is_factory to true in mk_payload

Bug: b/185537600
Test: Test that ARCVM linkerconfig works with change
Change-Id: Ib46da894a1f89f8ca86cd7cf2cc7907a7ec56153
diff --git a/microdroid/payload/mk_payload.cc b/microdroid/payload/mk_payload.cc
index 33e91b9..fd1ce78 100644
--- a/microdroid/payload/mk_payload.cc
+++ b/microdroid/payload/mk_payload.cc
@@ -171,6 +171,7 @@
         auto* apex = metadata.add_apexes();
         apex->set_name(apex_config.name);
         apex->set_partition_name("microdroid-apex-" + std::to_string(apex_index++));
+        apex->set_is_factory(true);
     }
 
     if (config.apk.has_value()) {