amp-anim should show the fallback/placeholder when the user prefers reduced motion

### Description

amp-anim should show the fallback/placeholder when the user has [prefers reduced motion](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion) set?

Also maybe adding support for all Level 5 User Preference Media Features

### Alternatives Considered

None.

### Additional Context

None.

Start by locating the amp-anim implementation and any existing handling for user preference media features. Check the linked prefers-reduced-motion guidance, then verify that the fallback or placeholder is shown when that preference is enabled; support for other Level 5 features remains an open-scope question.

Written by the indexing model from the issue text.