Avoid the split drag-to-dismiss transition during the task veil animation.

When the task veil animation is playing, the underlying task's transition should jump-cut to its finished state to prevent flickering.

Bug: 402781774
Flag: EXEMPT bug fix

Test: 1. On a tablet, enter split-screen mode with two apps. 2. Tap the handle bar of the app on the right. 3. Select the "dismiss split" option from the menu. 4. Repeat these steps multiple times under various conditions to confirm there is no flickering.
Change-Id: Ied8842aff69834ac1fc0728c1a9914bc84be0d67