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/apex/Android.bp b/apex/Android.bp
index 21054fc..04fd5f6 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -205,11 +205,10 @@
   ],
 }
 
-filegroup {
+file_contexts {
   name: "com.android.virt-file_contexts",
-  srcs: [
-    "com.android.virt-file_contexts",
-  ],
+  defaults: ["contexts_flags_defaults"],
+  srcs: ["com.android.virt-file_contexts"],
 }
 
 filegroup {