Use virtual dispatch for `ThreadGroup.add(Thread)`.

The method is package-private and unlikely to be ever
overridden but we should use the correct dispatch anyway.

The non-virtual dispatch was introduced in
    https://android-review.googlesource.com/602174 .

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Ic0b37d632e2affdb14c630a2ea197336351d8345