[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.*}

首先定位 ListVector getWriter API 及其 UnionListWriter 实现。检查当前如何处理重复的 getWriter 调用，并找出现有的 ListVector writer 测试；完成的标准是重复调用返回同一个 UnionListWriter 实例，同时不破坏 writer 的行为。

由索引模型根据 Issue 内容生成。