kfileaudiopreview: don't autoplay media files by default

Summary:
Not only it's annoying, but it's also a potential security risk (basically, a variant of [1]).

[1]: https://scarybeastsecurity.blogspot.com/2016/11/0day-poc-risky-design-decisions-in.html

Test Plan: No longer plays media files when hovering them in the file picker.

Reviewers: #plasma

Differential Revision: https://phabricator.kde.org/D7587