Fix Settings Panel bug causing frozen overview.

Previously, toggling search panel would freeze Overview mode
until the user exited back to the regular launcher. Also, the current
page would be lost. Fix this by computing the new scroll bounds
immediately when the setting is toggled.

Change-Id: Ib3587cfcea61e15b3d8b280113ce986cc57f2f9d
(cherry picked from commit 3de1e805afc243c10721fb32c3eeac51e4d0bd33)