[Java] Fix splitAndTransfer for variable width vector

When we splitAndTransfer a vector, the value count to set for the target vector should be equal to split length and not the value count of source vector. 

We have seen cases in operator slike FLATTEN and under low memory conditions, we end up allocating a lot more memory for the target vector because of using a large value in setValueCount after split and transfer is done.

**Reporter**: [Siddharth Teotia](https://issues.apache.org/jira/browse/ARROW-2294) / @siddharthteotia

_{**Note**: *This issue was originally created as [ARROW-2294](https://issues.apache.org/jira/browse/ARROW-2294). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

在 Java 實作中搜尋 splitAndTransfer 和可變寬度向量類別，然後追蹤 FLATTEN 如何呼叫該操作。驗證傳輸後目標向量的值數量是否與 split 長度一致，並檢查相關的現有測試，或為減少配置的情況補充涵蓋率。

由索引模型根據 Issue 內容生成。