ICU-22920 Avoid "return by const value" antipattern

[Quuxplusone]

Returning a const-qualified prvalue doesn't do anything useful, but it does turn an assignment such as v = rb.getLocale(); from a move-assignment into a copy-assignment (because it's forbidden to move-from a const value, even if it's a const prvalue). Each affected site was diagnosed mechanically by my fork of Clang. E.g.:

warning: 'const' type qualifier on return type is a bad idea [-Wqual-class-return-type]
  391 | const Locale ResourceBundle::getLocale(ULocDataLocaleType type, UErrorCode &status) const
      | ^~~~~

Checklist

• Required: Issue filed: ICU-22920
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable