ART: Remove some unnecessary mutator lock annotations.

The StackReference<> pointer held by a Handle<> can be used
without holding the mutator lock. We already do that when
we copy Handle<>s around. Only accessing the actual content
of the pointed-to StackReference<> needs to be done while
holding the mutator lock.

Change-Id: I5f93bd7e277383192f1f16dff6883ecb26387414
4 files changed