Media Grid: 

* Use a `media.view.Label` instance in the toolbar for the screen reader text for the "all" dropdown.
* Adjust the spinner's priority.

Fixes #29026.

Built from https://develop.svn.wordpress.org/trunk@29438


git-svn-id: http://core.svn.wordpress.org/trunk@29216 1a063a9b-81f0-0310-95a4-ce76da25c4cd