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GH-43962: [Java] Consider warnings as errors for Adapter Module (#43963)

### Rationale for this change

This PR configs the build such that warnings are considered as errors in the Adapter module. And corresponding code changes have also been made. 

### What changes are included in this PR?

Adding flags to consider warnings as errors in javac and fixing the corresponding errors. 

### Are these changes tested?

Tested by existing test cases. 

### Are there any user-facing changes?

N/A
* GitHub Issue: #43962

Authored-by: Vibhatha Lakmal Abeykoon <vibhatha@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>