Use on_vendor VNDK APEX for non-REL build

For non-REL branch, move VNDK APEX to /vendor partition so we don't need
to update vendor.img prebuilt everytime there is a change in the VNDK
libraries.
For REL branch, the API/ABI surfaces of VNDK libraries are frozen so
don't need to move to /vendor partition.

Bug: 140136207
Test: Build on REL and non-REL branch
Change-Id: I55d1ba68de3a4468250bae1b5fbcba6f62c599c2