Fix the logic for selecting ABI dump paths

A library may have both VNDK variant and stubs. getRefAbiDumpFile should
differentiate its vendor and core variants.

Bug: 152277104
Test: Add dumps to platform/ and vndk/ ; make libselinux.vendor libselinux
Change-Id: Iad038cf4cd3eccc3dfbef13fab67da044498ce77
Merged-In: Iad038cf4cd3eccc3dfbef13fab67da044498ce77
(cherry picked from commit 27d235f9)