Fix Video Player problem with keyboard shortcuts

[remibetin]

• Resolves Standard keyboard shortcuts are not working wai-video-standards-and-benefits#127
  Source: if (audiodescribed) was always true because an array in Liquid is always truthy even when empty (see https://shopify.github.io/liquid/basics/truthy-and-falsy/). Replaced by if (audiodescribed.length > 0)

• Resolves flaky behaviour when clicking on the "Audio description button"
  Solution: add data-status attribute to the <button> element and not to one of its inner elements.

• Simplify the code overall (remove unused code, etc.)

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[remibetin]

QA done: ✅