Fix JDWP stack frame regressions.

The bug was that Dbg::GetThisObject didn't take a thread argument, so
always used the calling thread, which is never the thread whose frames
you actually care about.

I've also made the JDWP ThreadReference.Frames get all the frames at once
now, since we no longer have fast random access to individual frames.

Change-Id: I0ce2bd2088e11d5ea14f9785d7ca0e3890b3cb9b