The file system on an Android device is not encrypted. iOS does this by default. However note that once you are at runtime this is not really relevant anymore. They only protect against attackers who have gained access to the phone and have removed the storage for analysis.
On Android, applications are sandboxed and as long as you are not running a rooted device one application cannot access the other application.
Generally if you want to encrypt your database it should be done from the application itself. Furthermore your encryption key should be derived from a password the user needs to supply every single time. Caching the password should be done carefully as you could dump it at runtime. The password or key should not be stored on the device.
Note that there is no foolproof way to protect your data once an attacker has gained access to the phone. You can only make it a bit harder, but not impossible.