Avoid using ThreadList::Dump() in empty checkpoint timeout.

ThreadList::Dump() that uses a checkpoint to dump threads may get
stuck when an empty checkpoint gets stuck and isn't useful when
debugging an empty checkpoint timeout. Instead, directly use
Thread::Dump() for each runnable thread that isn't responding to an
empty checkpoint request.

Bug: 33006388
Bug: 12687968
Test: test-art-host with CC.
Change-Id: If07a205786d469eec9b1f587485c80f54fb84c4d