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GH-43479: [Java] Change visibility of MemoryUtil.UNSAFE (#43480)

### Rationale for this change

`MemoryUtil.UNSAFE` field is a public field which provides unrestricted access to `sun.misc.Unsafe` instance which may cause misusage and possibly JVM crashes.

### What changes are included in this PR?

Make the field (and other related fields) private and only allow indirect use of Unsafe through `MemoryUtil` methods

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No

**This PR includes breaking changes to public APIs.** 

Code using `MemoryUtil.UNSAFE` would break as the visibility of the field was changed to private
* GitHub Issue: #43479

Authored-by: Laurent Goujon <laurent@apache.org>
Signed-off-by: Dane Pitkin <dpitkin@apache.org>