Fix SnapResizeAppWindowWithButtonTest

Currently, the test is failing due to divider width not
being taken into account. Since, divider is not even shown
on a one-side snap window, it's also not posisble to
determine the width. Hence, an approximation approach seems
to be better here, i.e. snapped window still fills > 95% of
either side of the screen.

Bug: 377090229
Flag: EXEMPT test fix
Test: atest
PlatformScenarioTests:com.android.wm.shell.functional.SnapResizeAppWindowWithButtonTest

Change-Id: I60b846f6bfc4d6fee2fe6f70ae5d6c1b71c008f2