Make LauncherState transitionDuration overridable at runtime

This follows the pattern of most of the other state properties, and
allows us, for example, to extend the overview transition in 0-button
mode, but keep it shorter for other modes that don't travel as far.

Bug: 143361609
Change-Id: Ibf8142bf3f57bb73be826adb6f4a32c136ff56dc