Support app shared libs in /product and /product_services.

This whitelist controls which partitions support installing an app's
libraries in the partition's lib folder as opposed to embedding
them in the APK itself. Update it to also include /product and
/product_services.

Also add some clarifying comments.

Bug: 111797707
Test: in internal branch
Change-Id: I756af46cc9b718ca669dacc9d25364edddb295f4