I found one but I think it's only for the Shield specifically.
The only ones I know of that can do it are Plex and Kodi, but I'm tired of playing around Plex to avoid transcoding and Kodi's switching doesn't reliably work for some reason. It will be fine in one file then then a few hours later there will be judder and frame skipping.
Is there an app that can reliably do it for any video player?
I implemented auto frame rate switching in my little video player app: https://github.com/moneytoo/Player
I believe that at least Vimu player also supports that.