RFR: 8316150: Refactor get chars and string size

[wickund]

On Wed, 13 Sep 2023 01:24:05 GMT, Shaojin Wen <duke at openjdk.org> wrote:

> 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

Changes requested by wickund at github.com (no known OpenJDK username).

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PR Review: https://git.openjdk.org/jdk/pull/15699#pullrequestreview-1639806273