ART: Call ThreadGroup.add in Thread::FinishStartup

ART should add the main thread to the main ThreadGroup. Behavior
of the Thread constructor changed.

Bug: 37444210
Test: art/test/testrunner/testrunner.py -b --host -t 051
Test: m test-art-host
Test: m build-art-host && art/tools/run-libcore-tests.sh --mode=host
Change-Id: I92cf2f9a6c5c3fdf385eb7925addc38b64fa4d98