🐛 Fix active page race condition in Web Story

[ychsieh]

There are 2 known cases where the race condition happen:

1. The visibility of the story becomes inactive before the active page is set(~20k / day):
   
2. By tapping, keying down, or clicking on the navigational button, the navigation happens before the active page is set(~40k / day):
   

This PR adds the async handler to finish the operation once the active page is set, instead of failing the entire story directly.

Context: #38979

[amp-owners-bot]

Hey @gmajoulet, @newmuis! These files were changed:

extensions/amp-story/1.0/amp-story.js
extensions/amp-story/1.0/test/test-amp-story.js

[ampprojectbot]

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