Mobile: Add image previews to Image block "Replace Featured Image" prompt

[SiobhyB]

This enhancement request builds on the work done to implement a "set as featured" button in the image block's settings in wordpress-mobile/gutenberg-mobile#1011.

At the moment, a user is greeted with the following dialog when they click the "set as featured" button while their post already has a featured image set:

As suggested here by @iamthomasbishop, it'd be useful to provide users with more details about their current featured image, to prevent them having to go diving back into post/page settings. This could potentially work in a similar way to how we display a preview of an image's focal point in the Cover block settings (sans the crosshairs and top-right button):

[SiobhyB]

Developer Notes

The code for the dialogs currently live on the native side, in the following places:

Android

• showFeaturedImageConfirmationDialog call: https://github.com/wordpress-mobile/WordPress-Android/blob/develop/libs/editor/WordPressEditor/src/main/java/org/wordpress/android/editor/gutenberg/GutenbergEditorFragment.java#L371
• Strings: https://github.com/wordpress-mobile/WordPress-Android/blob/develop/libs/editor/WordPressEditor/src/main/res/values/strings.xml#L131

iOS

• showAlertForReplacingFeaturedImage call: https://github.com/wordpress-mobile/WordPress-iOS/blob/10e87f524f306920d6fb232fbfed9c4f63813f23/WordPress/Classes/ViewRelated/Gutenberg/GutenbergViewController.swift#L689

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To refactor the dialog, the code will need to be removed from the native side and recreated in the ImageEdit component. It'd most likely make sense to call it from the onSetFeatured function here, before the new featured ID is sent over the bridge via setFeaturedImage: https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/image/edit.native.js#L462