Android has a notification implementation that supports priority levels. (https://developer.android.com/guide/topics/ui/notifiers/notifications.html). This means that you can define different priorities for the notifcation. lowest means no notification and max is that you get a pop up/in from the top even in full screen apps (like a movie)

Its very disturbing to have that pop in when you are busy with other stuff. especially in the weekend while you are not working (mattermost is in my case work related). If you look at Telegram, you can choose your priority for the notification, or at least this is possible via the settings in android.
However, because mattermost do not have this kind of notification implementation (i guess its not 100% as the design guide then) you can't choose it via the settings in mattermost, but also not via the system settings). It would be very very usefull if this could be implemented, so we can tune when and especially how we want to be disturbed.. If you want an example, install telegram, it has the correct and useful way of implementing the notifications.