Accessibility: Media: Make screen readers announce the Media search results.

- adds audible messages via `wp.a11y.speak()` to announce the search results 
- removes a `keyup` event that was there only to support old browsers 
- removes `aria-modal="true"` from the media modal dialog as it prevents the ARIA live regions to be perceived by screen readers
- triggers the search only after 2 ASCII characters have been typed

Props anevins, antpb, aduth, loreleiaurora, afercia.
Fixes #47146.

Built from https://develop.svn.wordpress.org/trunk@46239


git-svn-id: http://core.svn.wordpress.org/trunk@46051 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media-template.php

@@ -187,7 +187,7 @@ function wp_print_media_templates() {
 
 	<?php // Template for the media modal. ?>
 	<script type="text/html" id="tmpl-media-modal">
-		<div tabindex="0" class="<?php echo $class; ?>" role="dialog" aria-modal="true" aria-labelledby="media-frame-title">
+		<div tabindex="0" class="<?php echo $class; ?>" role="dialog" aria-labelledby="media-frame-title">
 			<# if ( data.hasCloseButton ) { #>
 				<button type="button" class="media-modal-close"><span class="media-modal-icon"><span class="screen-reader-text"><?php _e( 'Close dialog' ); ?></span></span></button>
 			<# } #>