Customize: Improve Media control accessibility and compatibility for...

Customize: Improve Media control accessibility and compatibility for `settings` passed as arrays or as solitary `setting`.

* Eliminate Media control template from having dependency on `params.settings.default` for element ID, to fix compat with `params.settings` array or single `params.setting`. See #36167.
* Move description out of label and add `aria-describedby` to Media control's Select button. See #30738, #33085.
* Obtain notification container whenever content is (re-)rendered (such as for Media control). See #38794.
* Re-render notifications after control content is re-rendered, if control is in expanded section. See #38794.

Amends [41390].
See #36167, #38794, #33085, #30738.

Built from https://develop.svn.wordpress.org/trunk@42031


git-svn-id: https://core.svn.wordpress.org/trunk@41865 1a063a9b-81f0-0310-95a4-ce76da25c4cd