Adjust DEX file reading to follow layout changes in ART.

I have changed the root DEX debug descriptor in ART to more
closely follow the JIT debug descriptor. Add the appropriate
offsets to correctly fetch the linked list head for DEX files.

Test: testrunner.py -t 137
Test: libunwindstack_test
Change-Id: I90402befc88fec42658f7330d51ee79756a7f872