Attempts fixing DismissSplitScreenByDivider flakiness

For some reasons on cf, the position can not dismiss split sometimes
after the drag divider bar to exit.  This CL drags the divider bar to
the edge of display bounds to make sure the split is dismissed always
for this test case.

Bug: 288106527
Test: atest WMShellFlickerTestsSplitScreen:DismissSplitScreenByDivider
Change-Id: Iaf7b31f38603ba8c4f9ab028609342b11b4d07bd