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: I9d89912fa6f00b80ce0e1c866b7800c02d36ee8c