If you tap outside the drop down menu while its animating in, we should
animate it away smoothly. Previously, we jumped to the reverseCurve,
which made the menu disappear immediately. Now we hold the animations as
state, which means we keep their _curveDirection property and don't
switch curves unless the animation actually finishes.

Also, fix a subtle bug in CurvedAnimation whereby we'd never set the
_curveDirection if we didn't see a status change in the parent
animation. Now we initialize _curveDirection based on the current value
of the parent's status.

Fixes #4379