Allow dexlayout to handle duplicate code items.

Dexlayout now checks to see if a code item already exists at a
particular offset before trying to construct a new one. If so, the
existing one is returned instead.

Bug: 37287151
Test: mm test-art-host-gtest-dexlayout_test
Change-Id: Iea21df01033e127de011568e1fe66224e260e202
(cherry-picked from commit d212d5b4)