[iOS] Don't notify on new message when talking to each other in another (open) session

Summary:
Despite being online in an active chat session with a colleague in (on my side) electron, I get a notification for each instantly-read message on iOS.

Steps to Reproduce:
* Chat with Bob in a session on one of two (or more?) devices connected to your account
* Have Bob send your a message
* see/hear/feel your mobile client alerting after having received that new message

Expected Results:
Not getting a (intrusive) notification for each message on iOS, since I'm reading them instantly.

Actual Results:
Explain what actually occurred when steps above were executed. If you have screenshots, screenshots are great!
Please provide error messages and URLs as text whenever possible.

Notes:
Maybe we should send a message without notification first, since they do get marked read and hide themselves soon after being received on the mobile device.

Version Information:
Desktop Client:
Riot/Electron 1.6.0

Mobile Client:
Riot 0.11.2 on iOS 13.5b2

My server:
synapse v1.12.4