Fix SecurityException on Asus Fonepads

This is a bug of their ROMs but the documentation says:

> Must be required by a VpnService, to ensure that only the system can bind to it.
>
> Protection level: signature
>
> Constant Value: "android.permission.BIND_VPN_SERVICE"

And we have `android:exported="false"` set, so I think it's safe to remove this permission restriction.

src/main/AndroidManifest.xml

@@ -131,7 +131,6 @@
                 android:name=".ShadowsocksVpnService"
                 android:process=":bg"
                 android:label="@string/app_name"
-                android:permission="android.permission.BIND_VPN_SERVICE"
                 android:exported="false">
             <intent-filter>
                 <action android:name="android.net.VpnService"/>