Define ro.product.vndk.version

Use ro.product.vndk.version to show the VNDK version that the product
partition is using.
When PRODUCT_PRODUCT_VNDK_VERSION is set, add ro.product.vndk.version
in /product/build.prop.
If PRODUCT_PRODUCT_VNDK_VERSION is "current", ro.product.vndk.version
will have the VNDK version in PLATFORM_VNDK_VERSION. Otherwise, it
will have the value defined in PRODUCT_PRODUCT_VNDK_VERSION.

Bug: 144534640
Test: Check if /product/build.prop has "ro.product.vndk.version"
Change-Id: If5e7e3a6c155de45f88f68700f16175656896afe