VNDK snapshot may provide LLNDK stub libraries

VNDK snapshots newer than v30 have LLNDK stub libraries. In that
case, the vendor variant for the BOARD_VNDK_VERSION must not be
generated from the LLNDK modules. They are already provided by the
VNDK snapshots.
Generate the vendor variant of the BOARD_VNDK_VERSION only if
BOARD_VNDK_VERSION has an integer value less than or equal to 30.

Bug: 187963715
Bug: 181815415
Test: m nothing
Change-Id: I31e90f4d7e7678c45558f6bf987f5582a4c34f78