[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，并检查其包级方法及当前调用方。明确范围是否仅包括将这些方法设为 public，还是还包括将类重命名为 UnsafeJniWrapper；当约定的 API 可供外部包访问，并且仍适用于与 Gandiva 和 Drill 的集成时，即视为完成。

由索引模型根据 Issue 内容生成。