Move MethodHelper::GetReturnType to mirror::ArtMethod.

Also, fix missing handle in HasSameSignatureWithDifferentClassLoaders.

Change-Id: I9e1ffd09be950ecc8346fc3c485760d82d9ecab3
diff --git a/runtime/reflection.cc b/runtime/reflection.cc
index b57e48f..228d200 100644
--- a/runtime/reflection.cc
+++ b/runtime/reflection.cc
@@ -632,8 +632,8 @@
   }
 
   // Box if necessary and return.
-  return soa.AddLocalReference<jobject>(BoxPrimitive(mh.GetReturnType()->GetPrimitiveType(),
-                                                     result));
+  return soa.AddLocalReference<jobject>(
+      BoxPrimitive(Primitive::GetType(mh.GetMethod()->GetReturnTypeDescriptor()[0]), result));
 }
 
 bool VerifyObjectIsClass(mirror::Object* o, mirror::Class* c) {