RFR: 8305746: InitializeEncoding should cache Charset object instead of charset name

[Peter Hofer]

Store `Charset` object in `jnuEncoding` and use `String(byte[], Charset)` and `String.getBytes(Charset)` instead of passing the charset name.

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Commit messages:
 - 8305746: InitializeEncoding should cache Charset object instead of charset name

Changes: https://git.openjdk.org/jdk/pull/13499/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13499&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305746
  Stats: 49 lines in 1 file changed: 18 ins; 14 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/13499.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13499/head:pull/13499

PR: https://git.openjdk.org/jdk/pull/13499