I have successfully stored up to 0.3 BTC for 4 years in my smartphone on Electrum wallet. I know this is not the smartest thing to do but I haven't lost my coins because I don't download any app and I don't visit any strange website.
Since this is only my experience and I know I was probably lucky, I suggest you to use hardware wallet if you are looking for cold storage. Even if you will manage to keep your device secure, you have to consider accidental drops (never happened to me luckily in my life), theft, damage, software/hardware problems and etc... Just buy a good hardware wallet, secure your seed phrases and seriously take care of your security.
Accidental drops, damages, software/hardware issues and physical loss are not critical if you have safely stored your seed elsewhere, especially onto a non-digital medium like a simple sheet of paper for example. Only a theft could put your funds at risk, if you have no mean to access your back-up seed quickly. So I wonder why people are so scared they try to dissuade a random Korean(we are not talking about a poor guy from poor country) to store 4 mbtc on a smartphone into a Smasung wallet. Bitcoin has no chance to be adopted one day if people need to be computer scientists and to have an airgapped computer along with another one in order to use it, sadly. Losing 3.9mbtc is sad but it's not the end of the world if it happens(which is very unlikely btw)