vndk-def: Add GenericRefs.classify_lib().

This commit adds classify_lib() function which is a generalization of
is_equivalent_lib().  According to the input library argument, it will
run NEW_LIB, EXPORT_EQUAL, EXPORT_SUPER_SET, and MODIFIED.

Test: ./tests/test_generic_refs.py
Change-Id: I3073efe975be6e08ff67bb31d7fcb2eb8b556230