RFR: 8275342: Change nested classes in java.prefs to static nested classes

[Daniel Fuchs]

On Fri, 15 Oct 2021 18:51:44 GMT, Andrey Turbanov <duke at openjdk.java.net> wrote:

> Non-static classes hold a link to their parent classes, which can be avoided.

I usually don't review changes in this area of core-libs but these are simple enough and look reasonable.

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Marked as reviewed by dfuchs (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5971