RFR: 8316150: Refactor get chars and string size

[Shaojin Wen]

1. Reduce duplicate stringSize code
2. Move java.lang.StringLatin1.getChars to jdk.internal.util.DecimalDigits::getCharLatin1，not only java.lang, other packages also need to use this method

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Commit messages:
 - Revert FormatItem related changes
 - restore StringUTF16.getChars(int,int,int,byte[]) and StringUTF16.getChars(long,int,int,byte[])
 - bug fix FormatItemDecimal::prepend
 - fix build error
 - refactor & bug fix
 - remove unused throws
 - remove unused throws
 - refactor from #15836
 - bug fix for DecimalDigits#getCharsUTF16(int, int, byte[])
 - remove unused code
 - ... and 17 more: https://git.openjdk.org/jdk/compare/d3e82183...d4f55ddc

Changes: https://git.openjdk.org/jdk/pull/15699/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15699&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316150
  Stats: 620 lines in 11 files changed: 279 ins; 303 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/15699.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15699/head:pull/15699

PR: https://git.openjdk.org/jdk/pull/15699