[Java] UnionListWriter does not behave properly when multiple lists are written in the same list

if you run the following code:
```Java
ListVector listVector = new ListVector("list", allocator, null);
listVector.allocateNew();
UnionListWriter listWriter = new UnionListWriter(listVector);

listWriter.start();
listWriter.startList();

listWriter.list("a").startList();
listWriter.list("a").bigInt().writeBigInt(1);
listWriter.list("a").bigInt().writeBigInt(2);
listWriter.list("a").bigInt().writeBigInt(3);
listWriter.list("a").endList();

listWriter.list("a").startList();
listWriter.list("a").bigInt().writeBigInt(4);
listWriter.list("a").bigInt().writeBigInt(5);
listWriter.list("a").endList();

listWriter.endList();
listWriter.end();

listWriter.setValueCount(1);
```

listVector will contain:
```
[{"a":[1,2,3,4,5]}]
```
Instead of
```
[{"a":[1,2,3],[4,5]}]
```

**Reporter**: [Abdel Hakim Deneche](https://issues.apache.org/jira/browse/ARROW-307) / @adeneche

_{**Note**: *This issue was originally created as [ARROW-307](https://issues.apache.org/jira/browse/ARROW-307). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

从 reproducer 中展示的 UnionListWriter 和 ListVector 设置开始，然后运行示例，观察重复的 list("a") 部分是如何存储的。跟踪 writer 对嵌套列表的处理行为，并根据示例添加回归测试。当该值表示为 [{"a":[1,2,3],[4,5]}]，而不是一个被展平的单一列表时，即表示完成。

由索引模型根据 Issue 内容生成。