It seems that Facebook for Android is adding a new UI supporting a new feature related to ringtones. The new UI will allow to pick sound files from the device and set as ringtones.
Check the image in the tweet below that shows the new UI for setting sound files from the device as ringtones in Messenger.
This has been revealed by Jane on Twitter. She is known to reveal new features and changes under testing for popular apps. It also seems that this new UI will also allow picking notifications, though Jane has not dwelled on that in her tweet.