Enable vndk-ext for product variants

To build vndk-ext for product variants use `vndk.extends` property
with `product_specific: true` as for the vndk-ext for vendor
variants. For example:
cc_library {
    name: "libvndk_ext_product",
    product_specific: true,
    vndk: {
        enabled: true,
        extends: "libvndk",
    },
}

It will install the vndk-ext libs for product variants in
product/lib[64]/vndk/

Test: m nothing
Bug: 147778025
Change-Id: If1ee5be93c579abad302f44f18e6316f27e70019