Rename ObjPtr::Decode to ObjPtr::Ptr

Done to prevent ambiguity with ScopedObjectAccess::Decode.

Bug: 31113334

Test: test-art-host
Change-Id: I07a2497cc9cf66386311798933547471987fc316
diff --git a/runtime/handle.h b/runtime/handle.h
index c41010a..d33d4a6 100644
--- a/runtime/handle.h
+++ b/runtime/handle.h
@@ -134,7 +134,7 @@
   ALWAYS_INLINE T* Assign(ObjPtr<T> reference) REQUIRES_SHARED(Locks::mutator_lock_) {
     StackReference<mirror::Object>* ref = Handle<T>::GetReference();
     T* old = down_cast<T*>(ref->AsMirrorPtr());
-    ref->Assign(reference.Decode());
+    ref->Assign(reference.Ptr());
     return old;
   }