JavaObjectPtr is alias to unique_ptr<ScopedJavaGlobalRef<jobject>>. It causes confusing because it's a triple state variable:

1. unique_ptr contains null ScopedJavaGlobalRef.
2. ScopedJavaGlobalRef contains null jobject.
3. Non null jobject.

In most of the cases, we just need two state. It's possible to pass ScopedJavaGlobalRef without unique_ptr. So we can remove this type.