[Java] Implement splitAndTransfer for UnionVector

This method was never implemented, and currently is a no op. We should at least do the naive "copy" version of the method.

**Reporter**: [Steven Phillips](https://issues.apache.org/jira/browse/ARROW-241) / @StevenMPhillips

_{**Note**: *This issue was originally created as [ARROW-241](https://issues.apache.org/jira/browse/ARROW-241). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

首先定位 UnionVector.splitAndTransfer，并检查附近的向量传输实现，以了解预期的输入和输出。然后实现所请求的 naive copy 行为，并验证该方法是否传输了适当的值且保留了预期的向量状态；issue 中未指定具体的测试文件。

由索引模型根据 Issue 内容生成。