libselinux is LLNDK, not VNDK-core

libselinux has stable C APIs and therefore provides a stub library for
Mainline clients. It is not copied into any Mainline modules and the
only instance of the lib in /system/lib serves all of them.

However for vendors, we still had another copy of libselinux in VNDK
(e.g. in the VNDK APEX that is served via the path /system/lib/vndk).
Since the library is guaranteed to have stable C APIs, there is no
reason to keep the vendor variant of the library in VNDK. Keeping it in
VNDK would be just a waste of storage space.

This change removes libselinux from VNDK and marks it as LLNDK. The
single instance of if in /system/lib serves vendor processes as well as
Mainline modules.

Bug: 151303316
Test: m
Change-Id: I7106ac236770b55b147c8a7f524bc2eddbddedb1