Fix HandleScope with wrong thread error

Fix HandleScope with wrong thread error

Possibly fixes
+art F  5127  6995 art/runtime/handle_scope-inl.h:43]
Check failed: top_handle_scope == this
(top_handle_scope=0x2b449293c770, this=0x2b4492d6a0a8)
Seen in tests. A possible cause was that the thread dumping stacks
was running checkpoints on a suspended thread but putting a
HandleScope on that thread instead of itself. This isn't safe if the
suspended thread is doing HandleScope stuff in a non runnable state.

Change-Id: Icdaadabca5aff8d380d5cc9570b9e899fe790a62