Create only one vendor variant depending on path

Not all vendor modules are meant to be working with multiple versions of
vndk unmodified. This restricts all vendor or vendor_available modules
to only one variant. Modules under proprietary directories will only
have BOARD_VNDK_VERSION variant, while modules under AOSP directories
will only have PLATFORM_VNDK_VERSION variant.

Bug: 157106227
Bug: 157133296
Test: capture snapshot from R
Test: try building master with R snapshot
Change-Id: I4ebe1da8d887cd76722fa8ab5ae9305da09074d4