Use original dex file for retransformation.

The spec requires us to pass the dex file as it appeared before any
retransformation-capable agents had modified it to the
ClassFileLoadHooks when RetransformClasses is called. We do this by
saving the initial dex file bytes into the class as a byte[].

Bug: 32369916
Test: mma -j40 test-art-host

Change-Id: Ic6af3738cd2a831e91ba1144f502fa58b3c333e4