For long time now, I have installed my Electrum Wallet on my android device and I haven't gotten any issues with that. However, there's nothing wrong to install your Electrum wallet on your android phone but you need keep it offline, and how to do that is having two phones. After installing your electrum wallet in your android device, the other things you should do is to keep your seed phrase out of reach from others.
Whats the point of having a wallet app on your mobile phone and keeping it offline all the time? Moreover, whats the point of having a smartphone at all if you are going to keep it offline all the time? I just dont see it.
If you need a device that will serve as cold storage, I think a hardware wallet is a much better (and cheaper) solution.