pulse: Duck volume of audio streams for notifications when on a phone call

Without this patch notification sounds (such as the sms sound notification) during a call can be disruptive to said call.

Fixes issue #56. The change is largely unmodified from: https://gitlab.gnome.org/World/Phosh/phosh/-/issues/699.

I've tested this on my librem 5 and it works. I do wonder if universal access should be included (which isn't in this merge request) to be ducked as well.

Also sorry about my stupidity on the issues, it's been a long week...