[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.*}

Busca en la implementación de Java splitAndTransfer y las clases de vectores de ancho variable, y luego sigue cómo FLATTEN invoca la operación. Verifica que el recuento de valores del vector de destino después de la transferencia coincida con la longitud del split y comprueba las pruebas existentes pertinentes o añade cobertura para el caso de asignación reducida.

Escrito por el modelo de indexación a partir del texto del issue.