Use virtual dispatch for `ThreadGroup.add(Thread)`. am: 2c12ef8a1b

Original change: https://android-review.googlesource.com/c/platform/art/+/2307788

Change-Id: Idbe4123a021bd8e8a01b08769c4248f183948cc4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>