[BUGFIX] Ensure the variation used for the next song is valid.

[AppleHair]

When playing in story mode, PlayState assumes all the songs use the same variation, which is usually true, but not guaranteed. This PR makes PlayState use the getFirstValidVariation method to find the valid variation instead of using the currentVariation variable, which might not be valid.

[EliteMasterEric]

Aside from the fact that Story Mode generally isn't used for variations, the issue here is that if you DO use a variation, then getFirstValidVariation will not be correct as it will try to use the default variation first.

The correct variation should be determined by first checking if the variation+difficulty is valid for that song, and using getFirstValidVariation only if variation+difficulty isn't valid.