Datepicker: templates not updating on JSON-Response

## What's the issue?

On dynamically updating `src` the calendar view is not updated with given templates. Only after moving a month forwards or backwards and then going back the calendar view updates with new values.

## How do we reproduce the issue?

https://www.leonids.de/amp-date-picker.html

1. Change the date and press Enter.
2. See how the marked date is **not** updated
3. Go one month forwards
4. Go back to March
5. Watch the mark setting in place

HTML
```

⚡️

```
JSON
```
{
"templates": [
{
"id": "testTmpl",
"dates": ["2021-03-15"]
}
]
}
```

## What browsers are affected?

Tested on Chrome/Firefox/Edge -- Android/Mac/Win

## Which AMP version is affected?

amp-date-picker 0.1

Reproduce the behavior at https://www.leonids.de/amp-date-picker.html using the listed HTML and JSON response, then inspect the amp-date-picker implementation and its dynamic src update path. Done means changing the date refreshes the marked template immediately, without navigating to another month and back.

Written by the indexing model from the issue text.