Fix DragDividerToResize flicker tests

Update the proper steps number for testing split resizing.Refine the tests since only the secondary side will be invisible with screenshot/icon-view fade animation.

Bug: 278008901
Test: atest WMShellFlickerTests:DragDividerToResize
Change-Id: Idd3f40d0e4da3f55092be9661c61215585d6a68c