As already mentioned, JNI_OnLoad is not required. Here is your problem:
Unable to instantiate activity ComponentInfo {Com.example.NDKDemo / com.example.NDKDemo.NativeLib}: java.lang.ClassCastException: com.example.NDKDemo.NativeLib cannot be added to android.app.Activity
NDK, : com.example.NDKDemo.NativeLib android.app.Activity.
, AndroidManifest.xml, , Activity. NDKDemo, , , AndroidManifest.xml:
<activity android:name="NDKDemo" ... other options ... >