ART: Use proxy ArtMethod's data_ to store the interface method.

This immensely simplifies the interface method retrieval
and removes one dependency on dex_cache_resolved_methods_.
We may later consider removing that member if we deem the
memory savings worth the performance impact.

Test: m test-art-host-gtest
Test: testrunner.py --host
Test: testrunner.py --host --jit
Change-Id: Id76349c69e4c4dea4e3b297bd504db8f98f1b7cc