viewSelector: Hide workspace page after animating to app picker

WorkspacesDisplay relies on being hidden to disable workspace switches
by scrolling or panning. Usually viewSelector will hide the previous
page on page switch, but we currently miss the case when opening the
overview at the app picker, where the workspaces page is still shown
for the transition, but never hidden.
Fix this by calling hide() in addition to setting the opacity to 0 at
the end of the overview animation.

https://bugzilla.gnome.org/show_bug.cgi?id=737534