ART: Add ClassLoadCallback

Add callback for class-load and class-prepare events. Move Dbg
over. Add tests.

Bug: 31684920
Test: m test-art-host-gtest-runtime_callbacks_test
Test: art/tools/run-jdwp-tests.sh --mode=host
Change-Id: I871f6b3c54448fd6ece8d9a7571b2042be50d525