Set updatable=false explicitly.

Currently, this is the default so this is a no-op,
But the default is changing to true.

Bug: 180375550
Test: Treehugger
Change-Id: Ic09bad5a792e7c0c7613ed4debfff40627425aeb
diff --git a/apex/Android.bp b/apex/Android.bp
index d6dda13..4f7201a 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -6,7 +6,7 @@
     name: "com.android.virt",
 
     // TODO(jiyong): make it updatable
-    // updatable: true,
+    updatable: false,
 
     manifest: "manifest.json",