Open correct handle menu when clicked in split screen.

Fixes a bug where the wrong handle menu would open when clicking caption
handles in split screen mode. Also fixes a crash when the "Close" handle
menu button is clicked in split screen.

Bug: 269660879
Test: atest DesktopModeWindowDecorViewModelTests
Test: Manual; click split screen menus on all sides and confirm they
open correctly.

Change-Id: I605ab470925608a6627a3a77b95d90dd880c05fd