[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.*}

Start with UnionListWriter and the ListVector setup shown in the reproducer, then run the example to observe how repeated list("a") sections are stored. Trace the writer behavior for nested lists and add a regression test based on the example. Done means the value is represented as [{"a":[1,2,3],[4,5]}] rather than a single flattened list.

Written by the indexing model from the issue text.