Fix thread hang

- Primary problem was ScopedThreadListLock was releasing heap lock in constructor instead of destructor
- Secondary problem was ScopedThreadListLock should not be used with Mutex::Wait
- Added Thread.getStackTrace case to ThreadStress that reproduces YouTube problem
- Added Mutex::GetDepth and related methods that were useful in diagnoising this issue

Change-Id: I1bdc7245e9b411378b98f4dcf498ad66eb96366d