apex_vndk doen't need stub variants

This was revealed by the new check added in
Ib3c6efd16bdf13b59f79be66d4f9dba49100f6cc.

This was okay since the old implementation relied on the fact that impl
variant comes before stub variants.

To make the new check happy and keep the intention explicit, stub
variants are not added to VNDK apex.

Bug: 191770320
Test: m (apex_test)
Change-Id: I455f2c9b6bc471c579379286c0198f36d325adf6