Create StreamRestore channel if none exists
Summary:
We need one channel to be able to control the volume. This restores D7485 which was broken by D20450 while fixing a different issue.
This patch tries to combine the two by only creating a new channel if none exists and not messing with existing ones.
BUG: 407397
Test Plan: Can control the notification volume in the KCM now. The slider position matches the one in pavucontrol
Reviewers: Plasma, drosca, broulik, ngraham
Reviewed By: ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D28822