* add ktlint plugin to project and apply default code style * some manual adjustments, fix wildcard imports * update CONTRIBUTING.md * fix formatting
* migrate to RxJava3 * remove unused import
* convert EmojiPreference and EmojiCompatFont to Kotlin * move preference related to to dedicated preference package * update proguard-rules.pro * reformat & add comment * maintain disposable information in EmojiPreference instead of EmojiCompatFont