Accessibility: Improve keyboard accessibility avoiding confusing tab stops in the Media views.

When sections of the media modal are hidden out of view, their controls may be
still focusable and actionable. This completely hides these sections to avoid
unnecessary tab stops and potential confusion for assistive technology users.

Props SergeyBiryukov, afercia.

Fixes #30599.

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git-svn-id: https://core.svn.wordpress.org/trunk@39469 1a063a9b-81f0-0310-95a4-ce76da25c4cd