Describe the new feature or change to an existing feature you'd like to see

Make <amp-carousel> work the same as <amp-carousel controls> for AMP for Email.

Describe alternatives you've considered

Alternative: make controls mandatory for amp-carousel in the validator. This is not really ideal developer experience.

Additional context

Gmail on mobile has the swiping motion reserved for moving between emails and thus documents that users should always use controls on mobile. It seems many other mobile clients have the same problem.