PTT: Mute state can get confused if talking whilst calls are still being set up

[dbkr]

Describe the bug
It's possible to be receiving media from someone without it looking like anyone is talking if that person starts talking early enough in the call.

To Reproduce
Steps to reproduce the behavior:

1. Enter room on browser A
2. Enter room on browser B
3. Very quickly switch back to browser A and hold space
4. You hear nothing for a short period of time while the call is still connecting, then hear your audio without it looking like anyone is speaking

[robintown]

Fixed by matrix-org/matrix-js-sdk#2487