JDWP: fix Dbg::ResumeThread

A thread should be resumed if and only if it has been suspended by
the debugger. Therefore, we must check the debug suspend count, not
the suspend count (which includes debug suspend count). Otherwise we
could end up resuming a thread that has been suspended for another
reason (like a GC) but not by the debugger itself.

Bug: 27385848
Test: art/tools/run-jdwp-tests.sh --mode=host --variant=X64
Change-Id: I70ed6dfc233501e92332634650babf21ba911048