There may not be any changes necessary here after https://mattermost.atlassian.net/browse/MM-18335 is completed.
1. Log into MM from two separate devices, at least one of which is mobile
2. Close the app on the mobile app
3. Mark the channel as unread from the other device with at least 1 mention message being below the New Messages line
4. Reopen the app on the mobile device
Expected: The app badge should update to match the total number of unread mentions as seen on other devices