Fix ART test depending on libcore internals.

The test was accessing an internal field which is now blacklisted.
This CL changes it to use another's classes instance field that
is part of the SDK API.

Test: art/test/testrunner/testrunner.py -b --host -t 918
Change-Id: I8f825c51acb5ab646e88cc94b27d95496e2d6788
2 files changed