[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，並檢查附近的向量傳輸實作，以了解預期的輸入和輸出。接著實作所要求的 naive copy 行為，然後驗證該方法是否傳輸了適當的值並保留預期的向量狀態；issue 中未指定特定的測試檔案。

由索引模型根據 Issue 內容生成。