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
On iOS, the app badge should be updated to match the number of unread mentions on the left side of the channel header whenever a channel is marked as unread, even if it happens from another device and/or when the app is closed.
On Android, the app badge will not be updated, but the user should not receive any erroneous notifications when marking a channel as unread from another device.
This should be tested with the other Mark as Unread changes on that branch. This also requires that the push proxy is updated as well before it can be tested.