[Java][Memory][Core]: Initialize DEBUG variable in BaseAllocator class separately

### Describe the enhancement requested

As we were implementing Java Nullability in https://github.com/apache/arrow/pull/37723, we found that there are many variables that are all tied to the DEBUG variable.

Request: Improve the nullability code by creating a new private static inner class to hold all the fields.

### Component(s)

Java

Start with the BaseAllocator class and review the nullability work referenced in Apache Arrow PR 37723. Identify the fields tied to DEBUG and determine how they should be grouped in a private static inner class. Done means the requested separation is implemented without changing the intended allocator behavior.

Written by the indexing model from the issue text.