ART: Correctly handle temporary classes in class-load events (3/3)

When a temporary class is given out in a ClassLoad event, all stored
references need to be fixed up before publishing a ClassPrepare event.

This CL handles objects stored in the heap.

Bug: 31684920
Test: m test-art-host-run-test-912-classes
Change-Id: Ia0456c81fd848618e637b93301edf4dbc8d848f2