smart_keep does nothing

[po5]

track-switched event got removed in https://github.com/mpv-player/mpv/commit/f9fd50c6546f2fa510251267d6423d3f342ca115
It had been deprecated for a while but reimplementing this ourselves adds a lot of complexity since tracks briefly get set to null, and because we only want to save tracks the user explicitly switched to.

Working on a fix but it may take a bit.