ART: Call ThreadGroup.add for attached threads

When attaching threads to the runtime, call ThreadGroup.add to
let the thread's group know that the thread is now started. This
fixes incorrect internal accounting.

Test: art/test/testrunner/testrunner.py -b --host -t 169
Test: m test-art-host
Change-Id: I60362b6b53acf06b97779ea9b3b0bc6264f1dc2d