Reduce non call volume during phone call

Problem: When you're on a call and you receive a text message (or anything else that generates audio), it comes through just as loud as when you're not on a call and thus is quite painfully loud and disruptive.

Potential Solution: I don't know if phosh is the right place for it, but it would be nice if it could reduce the volume for all other programs not a part of a phone call.

One case to think about is when on speaker phone. As it should likely still reduce the volume of other programs as to not allow them to interfere with the call.

Moved from: https://gitlab.gnome.org/World/Phosh/phosh/-/issues/699