Clean up Handle usage.

Prefer using ConstHandle instead of Handle as function arguments
since you can't assign new references to ConstHandle which helps
prevent bugs.

Changed NullHandle to be a ConstHandle so that you can never modify
it to be a non null reference.

Change-Id: I81cb979f6f8d5b49e5614966a2caf28c3701dd4f
diff --git a/runtime/object_lock.cc b/runtime/object_lock.cc
index f7accc0..a2668ec 100644
--- a/runtime/object_lock.cc
+++ b/runtime/object_lock.cc
@@ -22,7 +22,7 @@
 namespace art {
 
 template <typename T>
-ObjectLock<T>::ObjectLock(Thread* self, Handle<T> object) : self_(self), obj_(object) {
+ObjectLock<T>::ObjectLock(Thread* self, ConstHandle<T> object) : self_(self), obj_(object) {
   CHECK(object.Get() != nullptr);
   obj_->MonitorEnter(self_);
 }