apex: set ":vndk" as required for vendor apexes

When a vendor APEX sets use_vndk_as_stable: true, then the apex doesn't
bundle VNDK libraries but instead it uses them from the "vndk" namespace
at runtime.

":vndk" is a pseudo-name for VNDK libraries, which is processed by
linkerconfig so that the generated linker configuration allows the
namespace for the apex to access VNDK libs.

Bug: 159576928
Test: test with a test-only vendor apex
      it runs okay with VNDK libs. (lsof shows that it opens libs from
      VNDK APEX)
Change-Id: I924ce0fea696162575f59589b8f858deddf436b3