apex_vndk uses vendor variants only

VNDK APEX is supposed to contain "vendor" variants of VNDK libraries.
This is different from normal APEXes which have "apex" variants.

Bug: 146758869
Test: build / flash / boot
Change-Id: I5e035678c337334092616b58d2e0e404788a6639
Exempt-From-Owner-Approval: Got ORV, but rebased with resolving merge conflicts.