[java][jersey] Remove double brace initialization and some more jersey cleanup

Created by: snago

Some general cleanups in the Jersey 2 & 3 client generators.

Mainly remove double brace initialization in generated code, otherwise it's mostly about making the generated code more readable.

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

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