Define product_available property

To make a module available to product variants, it must define
`product_available: true`. `vendor_available: true` will not create
product variants any more.
However, in this CL, we don't change the behavior of
`vendor_available` property. It still creates both variants. After we
update all Android.bp files that need to provide product variants
with `product_available: true`, we may upload the remaining patches.

Bug: 150902910
Test: lunch aosp_arm64-userdebug && m
Change-Id: I0fd5be7bbae2c45d5cab3c3c2ca49f53a9b6f975
diff --git a/cc/vndk_prebuilt.go b/cc/vndk_prebuilt.go
index dddd5ac..bb86717 100644
--- a/cc/vndk_prebuilt.go
+++ b/cc/vndk_prebuilt.go
@@ -34,6 +34,7 @@
 //     version: "27",
 //     target_arch: "arm64",
 //     vendor_available: true,
+//     product_available: true,
 //     vndk: {
 //         enabled: true,
 //     },
@@ -251,6 +252,7 @@
 //        version: "27",
 //        target_arch: "arm64",
 //        vendor_available: true,
+//        product_available: true,
 //        vndk: {
 //            enabled: true,
 //        },