Amp story auto-advance-after a background-audio skips the page on iOS

When setting the auto-advance-after to the pageID it should use the background-audio length to advance, but on iOS (tested on Safari) it auto-advances the page really fast. This makes it impossible to navigate backwards.

```html

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The background audio does load and play if the auto-advance is disabled on the page, so the problem should not be related to the audio file.

cc @ampproject/wg-stories

Start by reproducing the provided amp-story-page example on iOS Safari, comparing auto-advance-after="p8" with auto-advance disabled. Trace how the page ID and background-audio duration are handled, then verify that the page waits for the audio duration and remains navigable backwards.

Written by the indexing model from the issue text.