Fix EnterSplitScreenByDragFromTaskbar#cujCompleted

Set the appExistAtStart to false for splitScreenEntered while verify
the appWindow isn't contain at start.

Work with aa9b365d33d9e85, make the flicker app support multi-instance
in split screen.

Bug: 270770472
Test: atest EnterSplitScreenByDragFromTaskbar
Change-Id: I6ec79adf21f45cdb5089838c9acf1745b4dd5a83