moz-message-bar does not get announced on Orca screen-reader

Orca does not announce the moz-message-bar content as an alert. Covered in bugzilla bug 1895857.

This means that the download warning, which we want to convert to moz-message-bar in #41820 (closed), will not work well with Orca.

I'll see what works for us, and I'll push for a solution upstream as well.