[Java] Remove reader index and writer index from ArrowBuf

Reader and writer index and functionality doesn't belong on a chunk of memory and is due to inheritance from ByteBuf. As part of removing ByteBuf inheritance, we should also remove reader and writer indexes from ArrowBuf functionality. It wastes heap memory for rare utility. In general, a slice can be used instead of a reader/writer index pattern.

**Reporter**: [Jacques Nadeau](https://issues.apache.org/jira/browse/ARROW-7494) / @jacques-n
#### PRs and other links:
- [GitHub Pull Request apache/arrow#6133](https://github.com/apache/arrow/pull/6133)

_{**Note**: *This issue was originally created as [ARROW-7494](https://issues.apache.org/jira/browse/ARROW-7494). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Empieza leyendo ArrowBuf y su herencia de ByteBuf para identificar la funcionalidad de los índices de lectura y escritura descrita en la issue. Sigue el uso de la API relacionada antes de decidir el alcance; se considera terminado cuando ArrowBuf ya no expone esa funcionalidad de índices, mientras el comportamiento restante del búfer sigue funcionando.

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