Feat: Add search and pinned selected values to MultiSelectListPreference

[Toldry]

Made this PR due to this comment.

Long MultiSelectListPreference lists (for example, 100+ keyboard layouts in Thumb-Key) are hard to navigate by scrolling alone. This PR adds optional parameters, all off by default, to all four overloads:

• searchable: shows a search field above the list.
• searchFilter: decides which values match. Default: case-insensitive substring match.
• searchPlaceholder, searchLeadingIcon, searchEmptyResult: composable slots for the field placeholder, the field icon, and the content shown when nothing matches. They are slots so the library adds no untranslated strings and bundles no icon.
• pinSelectedValues: repeats the values selected when the dialog opened at the top of the list, above a divider. They stay pinned when unchecked until the dialog is reopened.

With the defaults the dialog is unchanged. The new parameters come before item, so trailing-lambda callers still compile; binaries built against the old signatures need recompiling. preference/api/jvm/preference.api is updated accordingly.

The sample app gets a searchable preference demonstrating all options.

• Tests pass (the repo has no tests; apiCheck passes, and the library and sample compile. Tested manually in the Android sample on an emulator and in Thumb-Key)
• Appropriate changes to README are included in PR (none needed, README lists preference types only)

[Toldry]

Sample app preview:

Screen_recording_tiny.mp4

[Toldry]

Related: dessalines/thumb-key#2020

[zhanghai]

First of all, thank you for sending a PR! However, this isn't how the standard list preference works (as in androidx.preference), and different apps may have slightly different opinions on how it should be done (e.g. should unselected items immediately disappear, and should newly selected item immediately appear on top or not), so I think it should remain in the app that needs it instead of being upstreamed.