ART: Ensure correct thread state for stack trace collection

As the closure needs the mutator lock, the calling thread must hold
the mutator lock. Ensure that that is the case.

Test: m test-art-host
Test: art/test/testrunner/testrunner.py -b --host --trace --interpreter 911
Change-Id: I47d36abba83a0f38b398b9e3ef6f485b5e735622