RFR: 8316150: Refactor get chars and string size [v15]

[温绍锦]

> 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

温绍锦 has updated the pull request incrementally with two additional commits since the last revision:

 - bug fix for DecimalDigits#getCharsUTF16(int, int, byte[])
 - remove unused code

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15699/files
  - new: https://git.openjdk.org/jdk/pull/15699/files/3ace6b78..1ab0fbd0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15699&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15699&range=13-14

  Stats: 8 lines in 2 files changed: 0 ins; 7 del; 1 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