Make prebuilt_api test environment realistic

Previously, the fixture preparer for prebuilt_apis would add a
core-for-system-modules.jar file in every API directory even though
currently they only exist in the public API directories.

This change makes the test environment more realistic by only creating
them for the public API. Rather than hard code that into the test code
(which would duplicate the hard coding in the decodeSdkDep func) this
extracts a function that is used by both. That ensures that any changes
to that func will be reflected in both the test and runtime behavior.

Bug: 204189791
Test: m nothing
Change-Id: I346ac9c0dcf407c61de16b6027663a05821bcf62