[mediacontroller] Prefer plasma-browser-integration controls over browser built-in ones

When media controls from both plasma-browser-integration and the browser are present, only show the p-b-i one.

This avoids duplication and confusion.

Also, p-b-i controls work much better than the Firefox ones:
- thumbnails
- repeat controls
- seek bar
- artist name
- sometimes the Firefox controls don't even properly detect play/pause (try muting a YT video then opening another one also muted: Firefox thinks the new video is paused when it isn't)