Create product variant for product partition

When PRODUCT_PRODUCT_VNDK_VERSION is set to 'current', product
modules are enforced to use only VNDK libs from the system partition
as BOARD_VNDK_VERSION does to vendor partition.

Modules with 'vendor_available: true' create product variant as well
as core and vendor variants. The product variant as an image variant
is used for the modules in /product or /system/product.

It must not affect the current build behavior without
PRODUCT_PRODUCT_VNDK_VERSION set.

Bug: 134099726
Bug: 138966004
Bug: 144534640
Test: build without PRODUCT_PRODUCT_VNDK_VERSION set
Change-Id: I4d3585c110d84493e45bf76d550dc240bb26137f