Make locale_prefs private

[Manishearth]

Uses doc(hidden) accessors for getting/setting

Fixes #5786

[Manishearth]

---- tools/md-tests/src/lib.rs - tutorials::data_provider_md (line 23) stdout ----
error[E0616]: field `locale_prefs` of struct `icu_provider::hello_world::HelloWorldFormatterPreferences` is private
  --> tools/md-tests/src/lib.rs:64:79
   |
43 |     let loc = DataLocale::from_preferences_locale::<HelloWorldV1Marker>(prefs.locale_prefs);
   |                                                                               ^^^^^^^^^^^^ private field

---- components/datetime/src/neo.rs - neo::DateTimeFormatterPreferences (line 39) stdout ----
error[E0616]: field `locale_prefs` of struct `DateTimeFormatterPreferences` is private
  --> components/datetime/src/neo.rs:49:8
   |
12 | prefs2.locale_prefs.language = Language::try_from_str("fr").unwrap();
   |        ^^^^^^^^^^^^ private field

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0616`.
Couldn't compile the test.

What should we do about this?

[sffc]

This is for #5785

As far as DataLocale goes, we said:

• DataLocale moves to icu_locale_core::DataLocale
• LocalePreferences has two conversion functions into a DataLocale: to_data_locale_language_priority(self), to_data_locale_region_priority(self)
• DataLocale gets private fields