Fix janky movement when resizing split windows

This CL adds parallax movement to split windows when flingDividerPosition() is called at the end of a user movement.

We use a parallax effect when the user is dragging the divider around, but when the user releases touch on the divider, we animate it to the nearest snap point. Previously, this animation did not apply the same parallax effect, so the task contents appeared to jump around.

When the user is not controlling the movement, e.g. with split-to-side, we shouldn't use the parallax effect.

Fixes: 344708229
Test: Manually verified. User resize and auto-resize (split to side) both look good.
Flag: EXEMPT bugfix
Change-Id: Id6bf0acf6cfd50c8eee9fa95c926e49b7ef931fe