RFR: 8325590: Regression in round-tripping UTF-16 strings after JDK-8311906

[Roger Riggs]

Correct the result string coder of a string encoded using a CharsetDecoder with multi-byte encoded input.
Added tests for UTF16 strings and a regression test.

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Commit messages:
 - 8325590: Regression in round-tripping UTF-16 strings after JDK-8311906

Changes: https://git.openjdk.org/jdk/pull/17817/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17817&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325590
  Stats: 24 lines in 2 files changed: 18 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/17817.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17817/head:pull/17817

PR: https://git.openjdk.org/jdk/pull/17817