Clean up AtomicDexRefMap

Make ClassReference, TypeReference, and MethodReference extend
DexFileReference. This enables using all of these types as the key
for AtomicDexRefMap.

Test: test-art-host
Bug: 63851220
Bug: 63756964

Change-Id: Ida3c94cadb53272cb5057e5cebc5971c1ab4d366