[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.*}

Pesquise na implementação Java por splitAndTransfer e pelas classes de vetores de largura variável e, em seguida, rastreie como FLATTEN invoca a operação. Verifique se a contagem de valores do vetor de destino após a transferência corresponde ao comprimento do split e confira os testes existentes relevantes ou adicione cobertura para o caso de alocação reduzida.

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