RFR: 8300176: URLEncoder/URLDecoder static fields should be private static final [v2]

[Darragh Clarke]

> Made the `static` fields `private static final`, updated the naming as well to reflect this.
> 
> This meant I updated `URLDecoder` to set the `DEFAULT_ENCODING_NAME` itself since it had been previously getting it from `URLEncoder`.
> 
> Since these fields are only referenced from inside the classes nothing else needed updated

Darragh Clarke has updated the pull request incrementally with three additional commits since the last revision:

 - reduced bitset to 128 bits
 - formatting
 - implemented feedback from PR

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12122/files
  - new: https://git.openjdk.org/jdk/pull/12122/files/b08138d1..c276dce2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12122&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12122&range=00-01

  Stats: 16 lines in 2 files changed: 0 ins; 10 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12122.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12122/head:pull/12122

PR: https://git.openjdk.org/jdk/pull/12122