Lower the transition threshold for dropping on Workspace.

Old threshold: 0.5f
New threshold: 0.25f

The threshold is used when dragging to Workspace from any
drag source other than Workspace, including Hotseat. This
lowered threshold is more forgiving for faster drags.

Bug: 33210055
Change-Id: Ic121fb4b7caa3ea66abb48a8a3f3bd36f3365749