If you fully close the app (i.e. kill the process), it will default back to 12.5MP. If you just minimize the app and leave the camera app open in the background, which is fine to do, it will retain the higher resolution choice. There is no option I'm aware of to change the default to 50/200MP when you open the app after closing it down entirely. If you really want 50MP to be the default, there is no disadvantage to leaving the camera open indefinitely and that should do it for you.
Honestly you probably don't want it to default to 50MP anyway, unless you are always doing heavy cropping, making a print, etc., and even then you would be better off shooting a RAW image and processing that manually if your end goal was a print. 12.5MP is more than enough for most situations, but of course it's great to have the options. The 12.5MP mode also allows for 16:1 pixel binning which gives you the best low light performance on the main camera.