Fix EnterSplitScreenByDrag#cujCompleted

Set the appExistAtStart to false for splitScreenEntered while verify
the appWindow isn't exist at start for
EnterSplitScreenByDragFromAllApps. Update the target activity name for
EnterSplitScreenByDragFromNotification.

Bug: 270770472
Test: atest EnterSplitScreenByDragFromNotification#cujCompleted
      atest EnterSplitScreenByDragFromAllApps#cujCompleted
Change-Id: I693ee343c99be387aaff00e19fa2207f396f5806