[Task Manager] Enable "Mark applications that play audio" option only if plasma-pa is available

Everywhere else the code assumes plasma-pa might not be around but the config dialog would still enable
the check box unconditionally. Plasma-pa is not a dependency for Plasma-desktop and you can safely
uninstall it (if you prefer e.g. kmix), so we should cover this case.

Differential Revision: https://phabricator.kde.org/D4293