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/runtime/runtime.cc b/runtime/runtime.cc
index 6ef5f26..a2b462e 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -1101,6 +1101,7 @@
     Dbg::ConfigureJdwp(runtime_options.GetOrDefault(Opt::JdwpOptions));
   }
   callbacks_.AddThreadLifecycleCallback(Dbg::GetThreadLifecycleCallback());
+  callbacks_.AddClassLoadCallback(Dbg::GetClassLoadCallback());
 
   jit_options_.reset(jit::JitOptions::CreateFromRuntimeArguments(runtime_options));
   if (IsAotCompiler()) {