[Java] ListVector to return same UnionListWriter instead of returning new UnionListWriter

`The {color:#FF0000}getWriter{color} {color:#4c9aff}ListVector{color} API should return same {color:#4c9aff}UnionListWriter {color:#172b4d}instead of returning new {color:#4c9aff}UnionListWriter{color} everytime.{color}{color`}

**Reporter**: [Shivam Kapoor](https://issues.apache.org/jira/browse/ARROW-17209)

_{**Note**: *This issue was originally created as [ARROW-17209](https://issues.apache.org/jira/browse/ARROW-17209). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by locating the ListVector getWriter API and its UnionListWriter implementation. Check how repeated getWriter calls are currently handled and identify existing tests for ListVector writers; done means repeated calls return the same UnionListWriter instance without breaking writer behavior.

Written by the indexing model from the issue text.