[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.*}

Inizia individuando JniWrapper e verificando i suoi metodi a livello di package e gli attuali chiamanti. Chiarisci se l'ambito consiste solo nel rendere pubblici questi metodi o anche nel rinominare la classe in UnsafeJniWrapper; il lavoro è completato quando l'API concordata è accessibile ai package esterni e rimane adatta all'integrazione con Gandiva e Drill.

Scritto dal modello di indicizzazione a partire dal testo della issue.