[amp-video] ♿ override aria-label with alt

[processprocess]

amp-video accepts alt but it's not being read by screen readers.
On a video element, screen readers will read aria-label but not alt.

• Propagates title attribute
• Sets alt as aria-label if aria-label and title is not specified

Contributes to #32493

[gmajoulet]

As you mentioned alt is not valid HTML but valid AMP. The correct HTML way for publishers would be to add either title or aria-label.

• aria-label is already propagated to the <video> element, but title is not, we should fix this
• propagating alt as aria-label sounds good as a fallback if no title or aria-label was already provided

[amp-owners-bot]

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.circleci/config.yml
build-system/pr-check/module-tests.js
build-system/pr-check/nomodule-tests.js

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build-system/tasks/e2e/package-lock.json
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build-system/tasks/visual-diff/package-lock.json
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extensions/amp-story/1.0/amp-story.js

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extensions/amp-story/1.0/amp-story.js
src/amp-story-player/amp-story-player-impl.js