Add fallback logic to cancel entering split screen

There are cases not having enough task changes to enter split screen.
Update to support canceling the enter-transition by reparenting tasks
back to display.

Fix: 237635859
Bug: 250853925
Test: atest WMShellFlickerTests
Test: enable shell transition and verify the following behaviors
      1. drag and drop Phone to enter split while Phone in front
      2. select "split right" on Phone icon and Phone thumbnail in
	 recents to enter split
Change-Id: I0fe8dfc0c46a8b1f5ee70d85359d2078c263ac80