Fix check failure in ConcurrentCopying::IssueEmptyCheckpoint().

Thread::DumpState() internally uses ScopedObjectAccess which expects
that the caller does not hold the mutator lock if its in a suspended
state or that the caller holds the mutator lock and in the runnable
state. Use ScopedObjectAccess instead on the caller side to match this
expectation.

Bug: 34321097
Bug: 33006388
Bug: 12687968
Test: test-art-host with CC.
Change-Id: I04698d21fb4820a938009c7caeb8e81721329007