Accessibility: Media: Hide the invisible "file input" on media views from assistive technologies.

The file input button is visually hidden, but was announced by screen readers in workflows unrelated to file uploads.
It is now hidden from assistive technologies by the means of an `aria-hidden` attribute, as its CSS display property must not be changed to make sure it still works on old browsers.

See #49753.
Fixes #47611.

Built from https://develop.svn.wordpress.org/trunk@47834


git-svn-id: https://core.svn.wordpress.org/trunk@47610 1a063a9b-81f0-0310-95a4-ce76da25c4cd