Applies two changes:
- Use `TransformImmediately` which achieves the same wanted transition behaviour without any issues.
- Move the transition behaviour override into `BeatmapListingSearchControl` in a nested subclass of `UpdateableBeatmapSetCover`.
Culls another non-thread-safe mutation of the `Playing` bindable.
It seems to be a weird vestige from an earlier revision of the old
"direct" panel, which relied on `DisposeOnDeathRemoval` to finish track
playback (and then was removed in
6c150c9ed7). The play button is no longer
responsible for managing preview track lifetime anyway;
`PreviewTrackManager`'s method are intended for that.
Revealed by the framework-side transform thread safety checks. `Stopped`
is even annotated as not being thread-safe (but was annotated as such
long after the class's nascence).
Not the cleanest solution, but works for now. Will eventually be
replaced after the header is updated to reflect the latest designs
(which keeps it on screen in all cases).
Closes https://github.com/ppy/osu/issues/10703.