[css-speech] Make `speak: auto` respond to `visibility`

[fantasai]

Issue reported by Matt King and Jesse Beach: the auto value of speak should respond to visiblity as well as display: none.

[Crissov]

… or should there be audibility?

[aaaxx]

Related: #560.

[bocampbell]

Passed around IBM Accessibility group. Positive feedback, no objections.

[fantasai]

This was resolved to accept in https://lists.w3.org/Archives/Public/www-style/2017Feb/0004.html