ART: Start RuntimeCallbacks

Add a central RuntimeCallbacks structure to handle certain interesting
runtime events.

In a first iteration, add ThreadLifecycleCallback with ThreadStart and
ThreadStop. Move Dbg over to ThreadLifecycleCallback.

Add a test.

Bug: 31684920
Test: m test-art-host-gtest-runtime_callbacks_test
Test: art/tools/run-jdwp-tests.sh --mode=host
Change-Id: Ie0f77739a563207bfb4f04374e72dc6935c40b4f