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

android/neverallow.go

@@ -102,6 +102,7 @@ func createTrebleRules() []Rule {
 			In("vendor", "device").
 			With("vndk.enabled", "true").
 			Without("vendor", "true").
+			Without("product_specific", "true").
 			Because("the VNDK can never contain a library that is device dependent."),
 		NeverAllow().
 			With("vndk.enabled", "true").