Stops including extra VNDK apexes if the VNDK current apex is on vendor.

This is needed for merged mixed-release builds where the vendor half on
release N contains VNDK current (vN) on /vendor. This change prevents
the system half on release N+k from placing another copy of VNDK vN
on /system_ext.

Test: Build and merge crosshatch with S system and R vendor, boot
Bug: 176993126
Change-Id: I5322931244e060b2e7c92d9578e5eb64b098d1d6