[Python][C++] Simplify/Validate PyObject refcount in udf.cc

### Describe the bug, including details regarding any error messages, version, and platform.

In udf.cc, there is interaction between C++ and Python where the C++ code holds a user defined Python object inside a Kernel and invoke it over input data.

Due to this interaction, there are a number of `Py_INCREF` call as well as using `OwnRef` to manage the refcount for the PyObject (which represents the user defined function. The current code for managing it is either over complicated and/or bug where the C++ code might be over increasing the refcount. While this is not a huge issue (it probably cause the Python function to never be garbage collected), it could use some clean up / documentation as least to improve maintainability.

Appendix:
* Related thread: https://github.com/apache/arrow/pull/35514#discussion_r1205081272

### Component(s)

C++

Start by reading udf.cc and the related discussion in apache/arrow#35514, tracing each Py_INCREF and OwnRef use around the user-defined function held by the Kernel. Done means the reference-count handling is simpler, documented where needed, and no longer over-increments the PyObject.

Written by the indexing model from the issue text.