[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.*}

從 reproducer 中展示的 UnionListWriter 和 ListVector 設定開始，然後執行範例，觀察重複的 list("a") 區段是如何儲存的。追蹤 writer 對巢狀清單的處理行為，並根據範例加入回歸測試。當該值表示為 [{"a":[1,2,3],[4,5]}]，而不是單一個被攤平的清單時，即表示完成。

由索引模型根據 Issue 內容生成。