Make original dex file be more deduplicated

Now when retransformClasses is called we will use the DexCache from
the transformed class to hold the original dex file. This means that
there should be fewer redundant copies of dex files lying around from
class redefinition.

Bug: 31455788
Test: mma -j40 test-art-host-gtest
Test: ./test/testrunner/testrunner.py --host -j40

Change-Id: I60150e436d79aabb36da31ec679cb7ec4c820573