[Files] Fix image intersection observer behavior

[jloleysens]

Summary

Occasionally the observer does not fire with a "visible" event for the image tag. This happened because the img tag was placed below the blurhash and could be pushed out of view in a component with overflow hidden (as is the case in the file picker #143111). The result (see screenshot) is that the image can get "stuck" on the blurhash.

This PR makes the image tag appear "on top" of the blurhash so that when the container is scrolled into view it will render correctly.

Original context: https://github.com/elastic/kibana/pull/143111/files#r999145694

Also removes the "lazy" option from the Image component since it should, preferably, always load lazily.

How to reproduce the issue

On main

1. Launch Kibana with --run-examples
2. Go to the developer examples section in the side nav and open the file picker app
3. Ensure that you have a bunch of files uploaded
4. Click on the "Select a file" button to open the file picker
5. Flip back and forth between pages, scrolling down.

The issue

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesUx)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: be10533

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
files	25.3KB	25.4KB	+105.0B

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
files	19.5KB	19.5KB	-6.0B

Unknown metric groups

API count

id	before	after	diff
files	271	270	-1

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[Dosant]

Tested, works well!

Example app looks nice!