In the Android TV app, there is just a global stretch-to-fit option. I only need stretch-to-fit occasionally for ripped or downloaded recordings. I wish there were an a per-video (or per TV show) option to select stretch-to-fit. It's a bit inconvenient when starting to watch something and having to back out all the way to the global settings to turn stretch-to-fit on or off.
(Even better would be if this were somehow automatic. For DVDs, the players somehow figure it out. For my ripped videos that need stretching, VLC automatically shows it in the correct aspect ratio. I don't know how either of them detect the need.)