RFR: 8342650: Move getChars to DecimalDigits

[Shaojin Wen]

Move getChars methods of StringLatin1 and StringUTF16 to DecimalDigits to reduce duplication

1. HexDigits and OctalDigits also include getCharsLatin1 and getCharsUTF16
2. Putting these two methods into DecimalDigits can avoid the need to expose them in JavaLangAccess
3. Eliminate duplicate code in BigDecimal

-------------

Commit messages:
 - fix Helper
 - fix Helper
 - fix Helper
 - unsafe putByte
 - remove digitPair
 - fix import
 - remove JLA
 - remove unused code
 - add comments
 - move getChars to DecimalDigits & deduplicate

Changes: https://git.openjdk.org/jdk/pull/21593/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21593&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342650
  Stats: 710 lines in 11 files changed: 334 ins; 352 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/21593.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21593/head:pull/21593

PR: https://git.openjdk.org/jdk/pull/21593