I think it's best to let the user decide how he wants to secure his backup file. It's not the developers responsibility. Do you know of any other app that spits out encrypted backup files ? NZB360 should just create the backup file and the user must decide if and how he wants to encrypt/upload it.
This way nobody will complain about the encryption used. People who know what they are doing are able to choose the encryption algorithms that they prefer. And they can keep them stored on the medium they prefer, be it a encrypted HDD or their cloud service.
And..not everybody feels the need for an encrypted backup. I for example do not have any NN providers setup in NZB360 so I'm not worried about someone stealing those credentials. I don't have any authentication setup for the services because they are not opened up to the WWW. I have local access, and remote via VPN. Much safer than opening up untested software (security wise) to the internet.
If you're worried about someone stealing your (plain text) backup file, look at it this way:
If someone has access to your device or cloud or NAS or whatever medium you stored it on, I think NZB360/NN should be the least of your worries.....
That said, Android already offers the option to encrypt your internal AND external storage but that's only active when the device is turned off.
If you feel your backup file has been compromised, simply generating new API keys and credentials will suffice.