[Java] FieldVector getFieldBuffers API should not set reader/writer indices

Per discussion .

The fact that we have reader/writer settings in `getFieldBuffers` is wrong. To clarify, `getFieldBuffers` is distinct from `getBuffers`. The former should be for getting access to underlying data for higher-performance algorithms. The latter is for sending the data over the wire. Seems we've mixed up use of both.

Currently in `VectorUnloader`, we used `getFieldBuffers` to create `ArrowRecordBatch` that’s why we keep writer/reader indices in `getFieldBuffers`, we should use `getBuffers` instead.

**Reporter**: [Ji Liu](https://issues.apache.org/jira/browse/ARROW-7539) / @tianchen92
#### PRs and other links:
- [GitHub Pull Request apache/arrow#6156](https://github.com/apache/arrow/pull/6156)

_{**Note**: *This issue was originally created as [ARROW-7539](https://issues.apache.org/jira/browse/ARROW-7539). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Commencez par VectorUnloader et comparez son utilisation de getFieldBuffers et getBuffers lors de la création d’instances de ArrowRecordBatch. Lisez la discussion sur l’API FieldVector liée dans l’issue, puis vérifiez que getFieldBuffers fournit les données sous-jacentes sans modifier les index de lecture/écriture, tandis que la sérialisation des record-batches utilise getBuffers.

Rédigé par le modèle d'indexation à partir du texte de l'issue.