RFR: 8339017: Make a couple of fields in DoubleByte static

[Andrey Turbanov]

3 fields in `sun.nio.cs.DoubleByte.{Encoder,Decoder}` could be made 'static':
1. SS2
2. SS3
3. MAX_SINGLEBYTE

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Commit messages:
 - [PATCH] Make a couple of fields in DoubleByte static

Changes: https://git.openjdk.org/jdk/pull/20329/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20329&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339017
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/20329.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20329/head:pull/20329

PR: https://git.openjdk.org/jdk/pull/20329