I am in a problem - i need to read SecretKey from Android APK (e.g. key.keystore), for my app it has to be read as a FileInputStream, but from assets I am getting only input
I don't think this is possible because a FileInputStream is simply a InputStream for Files. It does the job of getting an InputStream from the defined file for you, after that you're working with a normal InputStream.
Android already did this Job for you. So why do you need a FileInputStream?