[Java] Make JniWrapper native method be public

The goal is to integrate Gandiva into apache Drill project. Now drill and arrow has some differences at the column in memory representation. Drill has a 2.0 plan to integrate arrow. Now I want to do some prior work to integrate arrow lib into drill project. The first thing is to make JniWrapper's package level methods be public ones. Maybe we can rename this classes as UnsafeJniWrapper like java's Unsafe class to allow others to invoke these methods directly.

So what's your opinion about this advice ? If ok , I will submit a corresponding PR.

**Reporter**: [Weijie Tong](https://issues.apache.org/jira/browse/ARROW-4819)

_{**Note**: *This issue was originally created as [ARROW-4819](https://issues.apache.org/jira/browse/ARROW-4819). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Comienza localizando JniWrapper y revisando sus métodos a nivel de paquete y sus invocadores actuales. Aclara si el alcance consiste únicamente en hacer públicos esos métodos o también en cambiar el nombre de la clase a UnsafeJniWrapper; se considera terminado cuando la API acordada es accesible para paquetes externos y sigue siendo adecuada para la integración con Gandiva y Drill.

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