[w3c/manifest] beforeinstallprompt not fire (#579)

I want to add to my app "Add To home screen banner", I went through the manual
https://developers.google.com/web/fundamentals/engage-and-retain/app-install-banners/#testing-the-app-install-banner
First I registered a sw in my index.html file
```
        navigator.serviceWorker.register('service-worker.js')
                .then(() => {
                  console.log('Service worker successfully registered.');
                })
                .catch(err => console.log("There is an error registering SW."))
```
Then I made a manifest file
```
{
  "short_name": "RUNtrix",
  "name": "RUNtrix",
  "icons": [
    {
      "src":"/img/logo-96x96.png",
      "sizes": "96x96",
      "type": "image/png"
    },
    {
      "src":"/img/logo-144x144.png",
      "sizes": "144x144",
      "type": "image/png"
    },
    {
      "src":"/img/logo-192x192.png",
      "sizes": "192x192",
      "type": "image/png"
    }
  ],
  "start_url": "/dashboard/day",
  "background_color": "#ffffff",
  "theme_color": "#536878",
  "display": "standalone"
}
```
Also I added eventListener in order to call prompt() in another place 
```
window.addEventListener('beforeinstallprompt', (event) => {
                    console.log('beforeinstallprompt fired index'); // It doesn't show at all
                  });
```
So it work's if I fire an event manually. But when I launch the site (served on https) from mobile devices it doesn't work.
Is it a bug? Or am I doing smth wrong?

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