[Java] Add missing imports for Set in java clients

Created by: languitar

In case of array properties declared with uniqueItems: true, the generated Java code uses Sets but imports are not present for them in the Api and Test classes.

Maybe this can be considered for a bugfix release also for older releases?

CC @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @nmuesch

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