[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.*}

VectorUnloaderから始め、ArrowRecordBatchインスタンスの作成時におけるgetFieldBuffersとgetBuffersの使用方法を比較します。issueにリンクされているFieldVector APIの議論を読み、そのうえで、getFieldBuffersが読み取り/書き込みインデックスを変更せずに基盤となるデータを提供する一方、レコードバッチのシリアライズではgetBuffersが使用されることを確認します。

索引モデルが issue の本文から書いたものです。