Don't hold any lock when visiting classes from JDWP

Computes reference type ids of all loaded classes without holding the class
linker lock. Because computing the JDWP reference type id can cause thread
suspension, we can't hold any lock. This is detected in debug build (using
libartd.so) and causes an abort.

Also adds missing thread safety annotations related to ObjectRegistry::lock_.

Bug: 17305632
Bug: 16720689

Change-Id: Id2cbd56624499c09e4755aaef7a9b15d2bdf4c91