[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.*}

まず JniWrapper を見つけ、そのパッケージレベルのメソッドと現在の呼び出し元を確認します。対象範囲がそれらのメソッドを public にすることだけなのか、クラス名を UnsafeJniWrapper に変更することも含むのかを明確にします。合意した API が外部パッケージから利用でき、Gandiva および Drill との統合に引き続き適していれば完了です。

索引モデルが issue の本文から書いたものです。