Fix crashes in NotifyByAudio when closing applications

Summary:
We have a race between close() and onAudioFinished() that resulted in the same
Phonon::MediaObject being added twice to m_reusablePhonons, which would result in a crash

BUGS: 380114

Reviewers: #frameworks, cullmann, rjvbb, apol

Reviewed By: apol

Subscribers: cfeck, rjvbb, mpyne

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D11891