release_config: make the code more sharable

This will allow the `build-flag` command to use release_config_lib code.

Bug: 328495189
Test: manual
Ignore-AOSP-First: cherry-pick
Merged-In: If068597b1d68c52c941788931997b7f77c73d2b3
Change-Id: If068597b1d68c52c941788931997b7f77c73d2b3