MessageTray: wait until the tray is hidden to switch styles

MessageTray: wait until the tray is hidden to switch styles

Switching style on Overview::hiding creates a weird effect, as the noise
texture is shown while the overview is still visibile. Instead, wait for
the tray to be fully hidden, then apply the new style.
As now the switch is invisible, there is no need for the transition
(which introduced the same problem on overview showing)

https://bugzilla.gnome.org/show_bug.cgi?id=689091