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The application presented in this blog uses the android.permission.WRITE_SETTINGS that, starting from Android v6.0 Marshmallow (API level 23), requires the application to be system or signed to be able to run without user intervention.
There's a workaround that can be used targeting API level 22. In this way the application can still use the old permission model and it can write into the settings.
However, because Google already announced that from August 2018 new application will need to target API level 26 to be accepted into the Play Store, this is a short lived solution. It's only a viable solution if your application is never going to be included in the play store.