ART: Fix RequestSynchronousCheckpoint

Fix the known races in the code by requiring more locks on entry.
This helps ensure there is no time in the caller where the thread
can die while requesting the checkpoint.

Fix up GetStackTrace, GetFrameCount and GetFrameLocation.

Bug: 62353392
Test: m test-art-host
Test: art/test/testrunner/testrunner.py -b --host -t 911
Change-Id: Ia0c5e920599b58098dc4b3a3d09c5284045f2947