Set windowing mode after animation on cancel transition.

Moves the windowing mode change to fullscreen after the
TRANSIT_CANCEL_ENTERING_DESKTOP_MODE animation finishes. This prevents
the home background from appearing at the start of the animation.

Bug: 280826960
Test: Manual, drag tasks from fullscreen, then move it back to status
bar in the same drag.

Change-Id: I3281fff75d812988952784c81ddc5ba1f3350ed4