Block Fields: Cover block issues

This tracks some issues with the cover block related to the 'Block Fields' experiment.

A prerequisite for testing is to enable the 'Block Fields' experiment on the Gutenberg experiment page.

1. backgroundType not set correctly

After #74575, there's a small issue with the cover block. The backgroundType property is not set correctly when adding a video using the media field in the inspector.

I believe this line is the problem: https://github.com/WordPress/gutenberg/blob/b06f167d4ecf1b38d22da21f58d5d0911fd56c3c/packages/block-library/src/cover/index.js#L88

It should read:

backgroundType: value.mediaType, 

It's probably worth rolling this fix into other cover block fixes.

2. Overlay opacity and text color (isDark) are not set correctly when adding new images

Steps to reproduce:

1. Add a cover block
2. From the inspector content tab, upload an image
3. Note that the image uploads isn't visible in the block. In the style tab the block still has 100 opacity

Cause: I think that the issue is that the block has some internal logic for calculating the dimRatio and isDark - https://github.com/WordPress/gutenberg/blob/b06f167d4ecf1b38d22da21f58d5d0911fd56c3c/packages/block-library/src/cover/edit/index.js#L194-L261

This code executes whenever uploading new images from the block, but not if some external code (like the Block Fields) tries to set a new image. A fix might be to move the code into a useEffect.