Add mock leashes for dimLayer and stage roots for 2 app flex split

Bug: 401375944
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: atest WMShellUnitTests:com.android.wm.shell.splitscreen.StageCoordinatorTests
Change-Id: I3536f5d141b18787a981ab044026903127db8074