Lock thread list, show join target

There were some paths to "thread list lock must be held" functions that
weren't locking the thread list.  We now do trylock/unlock in dumpFrames
to ensure that doesn't happen.

Also, append "tid=N" to the "waiting on" line in stack trace output
when we're waiting on a VMThread object (i.e. we're in Thread.join()).

Bug 2827015 (for the thread list lock).

Change-Id: I05aa27dd440c671b22ef7cfe47e31dfe54b0ed2d