Fix icon positioning for flexible split in Overview

In flexible split ratios like 90:10 and 10:90, we hide the Overview icon of the smaller app completely. This resulted in a bug where the single remaining visible icon was positioned badly.

Fixed this by adding a check in updateIconPlacement() so that we can skip the two-icon positioning and center the visible icon alone. Also fixed the cases where split select is initiated on a split tile (from Taskbar). Now the correct icon should be shown and centered.

Fixes: 391865942
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Single icon is positioned correctly in the middle. When split select is initiated, the right icon is shown and centered. When app chip menus are enabled, the (existing) correct behavior is not changed.
Change-Id: I79842222fc325a7661cbabbb54d277389a317504