RN: Update app badge when marking a post as unread from another device

Description

There may not be any changes necessary here after https://mattermost.atlassian.net/browse/MM-18335 is completed.

Test steps:
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

QA Test Steps

None

Status

Mana

2

Assignee

Harrison Healey

Reporter

Harrison Healey

Epic Link

Fix versions

Mattermost Team

Apps

Labels

None

GitHub Issue

None