Remove using mirror::* in jni internal.

Cleans up the code since it helps make it easier to spot moving GC
related bugs.

Also fixed moving GC bugs in MonitorEnter, and FindFieldID.

Change-Id: I794e4fbcc2010875fc6b299c6d4c9fb0f071b71a
diff --git a/runtime/mirror/object-inl.h b/runtime/mirror/object-inl.h
index b994354..2392561 100644
--- a/runtime/mirror/object-inl.h
+++ b/runtime/mirror/object-inl.h
@@ -60,8 +60,8 @@
   return Monitor::GetLockOwnerThreadId(this);
 }
 
-inline void Object::MonitorEnter(Thread* self) {
-  Monitor::MonitorEnter(self, this);
+inline mirror::Object* Object::MonitorEnter(Thread* self) {
+  return Monitor::MonitorEnter(self, this);
 }
 
 inline bool Object::MonitorExit(Thread* self) {