Error if vendor apex adds an LLNDK library

This is a common mistake when creating a vendor apex. They often try to
list all dependencies. Adding an LLNDK results in putting a stub library
in APEX, which simply doesn't work.

This change prevents stubs from being added to APEX.

Bug: 314033460
Test: go test ./apex
Change-Id: Ic3365047028d9ab6f06992d52aece5a3890177bb