Fix app asking for credential on every startup

app/src/main/java/com/cappielloantonio/play/interfaces/SystemCallback.java

@@ -0,0 +1,10 @@
+package com.cappielloantonio.play.interfaces;
+
+import java.util.List;
+
+public interface SystemCallback {
+
+    void onError(Exception exception);
+
+    void onSuccess(String token, String salt);
+}