ART: Fix potential deadlock in class-load events

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 fixes a potential deadlock when multiple classes are loaded
at the same time.

Bug: 31684920
Test: m test-art-host-run-test-912-classes
Change-Id: Iccabd8c17838f686a81a825badc7c2b1a1589708