Enforce adb_keys to be installed in product partition

adb_keys module type installs the output file into product partition
regardless of the partition specific property. To translate this logic
into product packaging with Soong, this change enforces adb_keys module
type to specify `product_specific` to true.

Test: m soong_generated_product_filesystem_test
Bug: 376515714
Change-Id: I23337f07b548caaf9361fbea525940394ffbcecc