fix(material/expansion): able to tab into descendants with visibility while closed (#24045)
fix(material/expansion): able to tab into descendants with visibility while closed (#24045)

The expansion panel sets `visibility: hidden` while it's closed in order to prevent users from tabbing into the content. This breaks down if a child has its own `visibility`, because it overrides the one coming from the parent. We can't use `display` in the animation definition, because it prevents the animations module from calculating the height when animating.

These changes add some CSS that will set `display: none` once the animation has settled.
crisbeto/material2,angular/components,crisbeto/material2,crisbeto/material2,angular/components,angular/components,angular/components,crisbeto/material2