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
diff --git a/test/XandY/Y.java b/test/XandY/Y.java
index ecead6e..2a1f036 100644
--- a/test/XandY/Y.java
+++ b/test/XandY/Y.java
@@ -14,4 +14,8 @@
  * limitations under the License.
  */
 
-class Y extends X {}
+class Y extends X {
+  static Z z = new Z();
+  static class Z {
+  }
+}