Fix DragDividerToResize flicker tests

Add a proper duration to dragging so it won't dismiss split screen
unexpectedly in transitions phase. Since both sides of split will be
invisible due to screenshot/icon-view fade animation, update the tests
to address the change.

Fix: 263213649
Test: atest WMShellFlickerTests:DragDividerToResize
Change-Id: I809bf7b66f065798b8b335ae5bd8f78ede5f2c83