Use WaitHoldingLocks in ReferenceProcessor::GetReferent.

The caller may be holding other locks, so we must use this or else
we get bad mutexes held errors. Also added a lock level for the
reference processor.

Change-Id: I364a303a4106982bd9bfaac5b116cea1638ea6d3
diff --git a/runtime/gc/reference_processor.cc b/runtime/gc/reference_processor.cc
index ef9e1d4..188b6b3 100644
--- a/runtime/gc/reference_processor.cc
+++ b/runtime/gc/reference_processor.cc
@@ -28,7 +28,7 @@
 
 ReferenceProcessor::ReferenceProcessor()
     : process_references_args_(nullptr, nullptr, nullptr), slow_path_enabled_(false),
-      preserving_references_(false), lock_("reference processor lock"),
+      preserving_references_(false), lock_("reference processor lock", kReferenceProcessorLock),
       condition_("reference processor condition", lock_) {
 }
 
@@ -71,7 +71,7 @@
         return obj;
       }
     }
-    condition_.Wait(self);
+    condition_.WaitHoldingLocks(self);
   }
   return reference->GetReferent();
 }