[Java][RestTemplate] useAbstractionForFiles - Use Resource interface instead of AbstractResource

[MosheElisha]

When useAbstractionForFiles=true, use Resource interface instead AbstractResource class to allow greater degree of freedom for the users calling the API methods.

A little improvement for #6715.

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[MosheElisha]

Adding the Java technical committee as requested - @wing328 @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @bkabrda. Please review.

[wing328]

Tested locally and the result is good.