RFR: 8351740: Clean up some code around initialization of encoding properties

[Stuart Marks]

Various behavior-preserving code cleanups related to system properties related to character encodings.

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Commit messages:
 - More minor cleanups, reversion of some extra changes.
 - WIP: add some non-null asserts and comments on sprops encoding members
 - A couple small tweaks.
 - Merge branch 'master' into JDK-8351740-encoding-properties-cleanup
 - Move platformProperties() native method declaration back to the bottom
 - 8351740: Clean up some logic around initialization of encoding properties

Changes: https://git.openjdk.org/jdk/pull/24463/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24463&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351740
  Stats: 51 lines in 5 files changed: 19 ins; 8 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/24463.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24463/head:pull/24463

PR: https://git.openjdk.org/jdk/pull/24463