Don't send push notifications to users who are clearly active on a device

This is a bug to track the various different bugs related to aspects of tracking what we've seen/read between clients:

High priority:
- [x] accurate synchronised unread state per-room (ie. number of regular messages unread or in vector, room boldness) (critical);
- [x] accurate highlight state per-room (subset of notif counts) (hopefully we have it already);
- [x] accurate badge counts on the favicon of the client

Lower priority:
- [x] accurate notification counts per-room (not that we show them in the UI currently, so it could be skipped)
- [x] self-read-up-to-marker
- [x] jumping to arbitrary message offsets (unrelated, basically)
- [x] push notification badge counts (not a blocker for v0)
- [ ] synchronising whether you push a user with whether they are currently active on a device, or have sent a read receipt, or whatever (not a blocker for v0)

Also need to show highlights somehow for rooms that are scrolled out of view on the RoomList or in a collapsed RoomSubList.

Immediate priorities:
Tracking unread per room, tracking highlights per room, ensuring they are synchronised between clients, and maintaining a favicon badge

### App specific issues

- [ ] https://github.com/vector-im/element-android/issues/3855