ART: Ignore repeated field indexes when loading a class.

This provides a deterministic behavior for classes with
duplicate field indexes in class_data_item and avoids
failures when verifying the field ordering in debug build.

Regression test not feasible without hand-edited dex file.
(Smali deduplicates field definitions.)

Bug: 21868015
Change-Id: I4f97ba005e063686f8f44248ed7f1286d987888a