Add product_available to libbinder_tokio_rs

Vendors already can use libbinder_tokio_rs on /vendor. This change makes
them available to /product as well, just like libbinder_rs.

More about product partitions:
https://source.android.com/docs/core/architecture/partitions/product-partitions

Bug: 336989719
Test: mm
Change-Id: I6c388c27b4fe6b7b1ff317bbfab424958fb6a6a9