This question is much more general than an apk, Python, or Kivy.
The short answer is always, you can't keep secure keys in a program.
What ever the language, where ever the software, eventually somebody will find the keys.
The only thing you can do is make it hard, not worth their time.
Keys could be provided externally at runtime, a password is a simple example.
Since crypto keys can be unfriendly to humans, consider keeping the crypto key on a server passed to the user when they login.
Security is a very specialized and complex issue, the above is to start you thinking.......