Temporarily hide split-screen divider during launch

This change ensures the split-screen divider bar is only visible once the split-screen tasks have finished launching. Previously, the divider could appear before the tasks were visible.

Bug: 431120034
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: 1. On a foldable device, create multiple split-screen pairs. 2. Open the Recents screen. 3. Launch a split pair that is not the most recent one. 4. Verify the divider bar appears at the same time as the apps.

Change-Id: I99e424977aedd7649c4a7dfd92bcb1284eef1b62