Revert "Revert "ART: Add ThreadGroup API support""

This reverts commit 87071bfb.

Add an ObjectLock, which corresponds to the synchronized(this)
implementation on the Java side. Wait for the expected five
child threads in the root group before running the actual
child test.

Bug: 31455788
Change-Id: Ib7a065d6a11f06f0325e3a8db040629f3ca69407
Test: m test-art-host-run-test-925-threadgroups