amp-install-serviceworker: Trigger event when service worker has been update has been installed

At the moment there is no way to alert the user when an updated service worker has been installed. Often pages inform the user of this with a prompt such as:


I suggest that `amp-install-serviceworker` be extended to add support for an `updatefound` event which can be triggered on the `amp-install-serviceworker` element.

https://github.com/ampproject/amphtml/blob/9e9554c19a232664fe299fd9195dada19f4f7781/extensions/amp-install-serviceworker/0.1/amp-install-serviceworker.js#L293-L303

Perhaps instead of `updatefound` it would be better to name it `updateinstalled` since as I understand we're primarily interested in when an `updatefound` event results in `'installed' === registration.installing.state` being true. So perhaps something like so:

```js
function install(win, src) {
return win.navigator.serviceWorker.register(src).then(function(registration) {
if (getMode().development) {
user().info(TAG, 'ServiceWorker registration successful with scope: ',
registration.scope);
}

reg.addEventListener('updatefound', () => {
reg.installing.addEventListener('statechange', () => {
if ( 'installed' === registration.installing.state ) {
const name = 'updateinstalled';
const event = createCustomEvent(this.win, `${TAG}.${name}`, dict({}));
Services.actionServiceForDoc(this.element).trigger(this.element, name, event, ActionTrust.HIGH);
}
});
});

return registration;
}, function(e) {
user().error(TAG, 'ServiceWorker registration failed:', e);
});
}
```

With something like this in place, an author could then show a notice using `amp-bind` like so:

```html
false

Update available!

```

(Maybe there should be an `AMP.reload()` action available as well.)

Thoughts?

Start with extensions/amp-install-serviceworker/0.1/amp-install-serviceworker.js around the linked registration code at lines 293-303. Review the issue's proposed updatefound/statechange flow and the existing action trigger pattern; done means the element exposes a documented event when an updated service worker reaches the installed state, with coverage added for that behavior.

Written by the indexing model from the issue text.