amp-selector: loses VoiceOver support when multiple attribute is added

### Description

When testing Chrome Version 91.0.4472.164 using an amp-select element with the `multiple` attributes added.

When selecting elements, I am only able to select one option and unable to navigate to the other options.
Once a selection has been made and I navigate out of the listbox I can no longer focus on it, the screen reader will just jump between the element before it and the element after it.

Tested in Safari and Firefox and it seems to be working as expected.

I initially found this issue looking at AMPforEmail but it appears to also be an issue in AMP HTML

### Reproduction Steps

- Open the amp by example link below in Chrome
- Open VoiceOver and navigate to the second listbox
- Make a selection
- Try and navigate to the other options
- Exit the listbox and try and navigate back to it

https://playground.amp.dev/?url=https%3A%2F%2Fpreview.amp.dev%2Fdocumentation%2Fexamples%2Fcomponents%2Famp-selector&format=websites&_gl=1*k04z35*_ga*Njg0NTYzOTUwLjE1ODMzNDQ5NDM.&mode=Responsive

### Relevant Logs

_No response_

### Browser(s) Affected

Chrome

### OS(s) Affected

macOS 11.4

### Device(s) Affected

macbook pro 15 inch 2018

### AMP Version Affected

_No response_