I'm not sure what the cause of the issue is, but I'm also not sure why
it wasn't using `DrawablePool` (was it not around back then?).
Either way, switching to a proper pool fixes things just fine.
Resolves https://github.com/ppy/osu/issues/25009.
Previously, non-pooled DHOs were immediately added as children of the HOC when Add is called. Also, non-pooled DHOs were always attached to the HOC as children.
New behavior is that non-pooled DHOs are only added after CheckChildLifetime, and only attached to the HOC while the DHOs are alive.
- LifetimeManagementContainer inheritance of HOC is removed, as it is now all DHOs are "unmanaged" (previously `AddInternal(false)`).
- The signature of `Clear` is changed, and it is now always not disposing the children immediately.