[Mobile] Importing a Media-Text block without isStackedOnMobile set defaults to the wrong value

[chipsnyder]

Describe the bug
When importing content to a post or page without isStackedOnMobile set will default to false where other instances default to true.

To reproduce
Steps to reproduce the behavior:

1. In the demo app set the initial HTML content to:

export default `
<!-- wp:media-text {"align":"full","mediaId":696,"mediaType":"image","imageFill":true} -->
<div class="wp-block-media-text alignfull is-stacked-on-mobile is-image-fill"><figure class="wp-block-media-text__media" style="background-image:url('https://a8ctm1.files.wordpress.com/2020/01/dane-deaner-lgjoc2cmff4-unsplash.jpg?w=683');background-position:50% 50%;"><img src="https://a8ctm1.files.wordpress.com/2020/01/dane-deaner-lgjoc2cmff4-unsplash.jpg?w=683" alt="" class="wp-image-696" /></figure>
<div class="wp-block-media-text__content">
<!-- wp:paragraph {"className":"has-large-font-size"} -->
<p class="has-large-font-size"></p>
<!-- /wp:paragraph -->
</div></div>
<!-- /wp:media-text -->

2. Run the demo app
3. Expect the Media-text block to be stacked:

Actual	Expected