Specify product_specific to true in frameworks-base-overlays

All required modules of this module installs to product partition.
Specifying this property also assures that (the required modules of)
this module is installed to the product partition for soong generated
filesystem partition module.

Test: m soong_generated_product_filesystem_test
Bug: 376502478
Change-Id: Iac84c3cd37b85cbdeb12ef200cece45d7f6250ed