implemented loading of files with KeepassXC Challenge (#4).

requires write support, handling of Challenge/Response errors (or user cancels). Caution: saving corrupts the file at the moment!

src/KeePassLib2Android/Cryptography/KeyDerivation/Argon2Kdf.cs

@@ -68,7 +68,10 @@ namespace KeePassLib.Cryptography.KeyDerivation
 			get { return "Argon2"; }
 		}
 
-		public Argon2Kdf()
+        public override byte[] GetSeed(KdfParameters p)
+        { return p.GetByteArray(ParamSalt); }
+
+        public Argon2Kdf()
 		{
 		}