0.25.0.1555 beta Telegram X supports displaying custom emoji in all places of the app.To get this version, you may subscribe to beta through this link and receive it like a regular update through Google Play.
Alternatively, you may download an APK from this official channel.
0.25.0.1555 also comes with many new features and bugfixes. You can the check source code and full list of changes on GitHub: #217, #205.