[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.*}

Commencez par localiser JniWrapper et examiner ses méthodes au niveau du package ainsi que leurs appelants actuels. Déterminez si le périmètre consiste uniquement à rendre ces méthodes publiques ou également à renommer la classe en UnsafeJniWrapper ; le travail est terminé lorsque l'API convenue est accessible aux packages externes et reste adaptée à l'intégration avec Gandiva et Drill.

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