[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.*}

首先定位 JniWrapper，並檢查其套件層級方法及目前的呼叫端。釐清範圍是否僅包括將這些方法設為 public，還是也包括將類別重新命名為 UnsafeJniWrapper；當約定的 API 可供外部套件存取，且仍適合與 Gandiva 和 Drill 整合時，即視為完成。

由索引模型根據 Issue 內容生成。