Slideshow Block: When using VoiceOver (screenreader) the focus automatically goes to autoplay slides

#### Steps to reproduce the issue

1. Add some images to the Media Library, be sure to add an alt text for each one
2. Create a new page or post and add the Slideshow block with the same images from step 1
3. Enable "Autoplay" on the Slideshow block settings. Publish.
4. Visit the published page or post and enable VoiceOver.

#### What I expected

For each slide to be described when I put my focus on the Slideshow using VoiceOver

#### What happened instead

VoiceOver automatically focuses on each slide even when you hadn't selected the slider. This is a bit frustrating since it will interrupt when you are navigating through other items on the same page or posts.

**Video**
In this video you'll notice I'm at the top of the page, I haven't started navigating down just yet and VoiceOver is reading the alternative text for each image in the slideshow as the autoplay runs:

https://cloudup.com/cG03s-MMhh6

#dogfooding