ART: Fix issues in RequestSynchronousCheckpoint

Do not run the checkpoint for another thread while being suspended.
Only suspend while waiting for that thread to become suspended.

After decreasing the suspend count, broadcast on the resume condition,
to tell the thread that it may progress.

Bug: 62353392
Test: m test-art-host
Change-Id: I63082b1fac5ec1db6ac31199548b9ae3cb30ea47