[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.*}

Start by locating UnionVector.splitAndTransfer and inspect nearby vector transfer implementations to understand the expected inputs and outputs. Implement the requested naive copy behavior, then verify that the method transfers the appropriate values and preserves the expected vector state; no specific test file is named in the issue.

Written by the indexing model from the issue text.