Use Gericop's preference library (#1471)

* New SimpleMenuPreference;
* More Oreo-like preference style;
* I'm tired of fixing all those bugs;
* Remove NumberPickerPreference which nobody likes.

mobile/src/main/java/com/github/shadowsocks/DialogFragment.java

@@ -1,15 +0,0 @@
-package com.github.shadowsocks;
-
-import android.app.Activity;
-
-/**
- * This helper class is used to solve Java-Scala interop problem. Use onAttach(Context) and remove this class
- * when minSdkVersion >= 23.
- *
- * @author Mygod
- */
-public class DialogFragment extends android.app.DialogFragment {
-    protected void superOnAttach(Activity activity) {
-        super.onAttach(activity);
-    }
-}