overview: Set _shown before calling _animateNotVisible

_hideDone checks _shown to determine if anything has shown the overview
while we hid it, and if so, shows the overview forward just in case.

In a local patch that called _hideDone immediately inside _hide for
testing, this broke. While we don't actually depend on this anywhere,
it doesn't hurt so that the next person to hack this up (perhaps me!)
doesn't get stuck debugging it for 20 minutes.