The main idea was to have simple app to turn on/off "Do Not Disturb" mode. I needed mini app accessed from screen widget to easily set quiet mode for important business meetings. For the first use, you'll have to set permissions to toggle "Do Not Disturb" mode and app will open settings screen for you. Just set app permission and click back. From that moment, app will be allowed to toggle quiet/normal mode.

In Android 6.0 (like my Samsung S7), app permission path is on the following location:

Settings -> Lock screen and security -> Other security settings -> Do not disturb permission

And is possible to customize how DnD mode will behave - like without any exceptions (as it's for Alarms). Just open the settings on the following path and set:

Settings -> Do not disturb -> Allow exceptions