Variable Speed improvements

Changes to variable speed settings, so that they apply dynamically.

Implemented the warning if you try to enable variable speed playback without having an appropriate library installed. 

PLEASE NOTE that the links to Google Play store only kinda work, because *something* in Car Cast is firing and causes Car Cast to redisplay itself, and close the Google Play app.

I suspect it's CarCast.onActivityResult

If you can't see what's causing this refresh issue, then I can just take out the links to the Play store, and user will have to do it manually. Very weird though!