JDWP: fix thread_list deadlock

Limits the scope of Locks::thread_list_lock_ locking in the debugger
so we do not try to lock it twice when creating a JDWP id (because
calling Object::IdentityHashCode may need to take the lock).

Bug: 20048099
Change-Id: I305dd72ccc4d2d007d1603b0d52bcfa94b6842a7
4 files changed