Flag-guard vfio_handler policies

vfio_handler will be active only if device assignment feature is turned
on.

Bug: 306563735
Test: microdroid tests with and without the flag
Change-Id: I5559dfca1a29852b65481c95f37edc9977ee9d7d
diff --git a/private/service_contexts b/private/service_contexts
index 898cb14..6d0851c 100644
--- a/private/service_contexts
+++ b/private/service_contexts
@@ -152,7 +152,9 @@
 android.service.gatekeeper.IGateKeeperService    u:object_r:gatekeeper_service:s0
 android.system.composd                    u:object_r:compos_service:s0
 android.system.virtualizationservice      u:object_r:virtualization_service:s0
-android.system.virtualizationservice_internal.IVfioHandler u:object_r:vfio_handler_service:s0
+is_flag_enabled(RELEASE_AVF_ENABLE_DEVICE_ASSIGNMENT, `
+    android.system.virtualizationservice_internal.IVfioHandler u:object_r:vfio_handler_service:s0
+')
 ambient_context                           u:object_r:ambient_context_service:s0
 app_binding                               u:object_r:app_binding_service:s0
 app_hibernation                           u:object_r:app_hibernation_service:s0