Label /vendor/priv-app as vendor_app_file

In P, we will be supporting privileged apps in vendor partition, thus
need to label /vendor/priv-app as vendor_app_file so that apps can exist
under the dir.

Bug: 35301609
Test: N/A since there is no /vendor/priv-app yet. Framework change
which is currently in the internal is required.

Change-Id: I86a765ef9da5267113e64a7cbb38ba0abf5c2835