Show warning when user tries to set notification icon to "None"

We should warn user that turning notification off will make the app work unstable.

Title: **Warning**

```
Please note, that if we remove the icon, Android could stop Adguard any time, which will result in loss of connectivity. We recommend you to use "Notification without icon" instead.
```

Buttons:
"OK" - switches to "Notification without icon"
"Change anyway" - switches to "None"
