[Task Manager] Don't delay audio indicator when task appears

When the task appears in Task Manager and is already playing audio, show the indicator right away.
There's no distraction from that since the task as a whole appears.
Only when the audio stream appears during the lifetime of a task, it is delayed to avoid distractions by short sound effects.

BUG: 390502
FIXED-IN: 5.13.0

Differential Revision: https://phabricator.kde.org/D10539