Revert "Revert "Change ClassExt.originalDexFile field to an opaque Object""

This reverts commit ac9ee270.

The Art makefiles got broken causing target tests to fail.

Reason for revert: Fixed issue with target tests

Bug: 31455788
Test: m clean && ./art/test.py --target -j4

Change-Id: Iff098638b23c4fdf84f9fc3d807a7468c6177883