Carousel stops after manual interference.

## Describe the new feature or change to an existing feature you'd like to see
The `amp-carousel` in autoplay stops permanently after manual interference i.e once the user tries to go to the previous or next slide by sliding or pressing the carousel arrows the autoplay stops.
The use case which I can think of is that there could be some readable content and the user is given as much time as they require to go through it. But once the user has passed that section, the autoplay should begin again.
The auto-play re-starts once the user switches between tabs.

## Describe alternatives you've considered
The `amp-carousel` can have a boolean **attribute** stating that should the autoplay begin once the user has passed or clicked outside that section. Or the autoplay should begin again once it re-enters the viewport.