Ensure WCT#setDisallowOverrideBoundsForChildren works for existing child

Even if the split-screen root task disallows its child tasks from
having override bounds, split-main and split-side tasks can still
have them. This is because WCT#setBounds directly updates the
Task’s overrideConfiguration.

This change ensures that child tasks cannot have override bounds,
and apply the behavior on the split-main and split-side tasks vs.
the split root task.

Bug: 427245857
Test: atest WMShellUnitTests:StageCoordinatorTests
Flag: com.android.wm.shell.split_disable_child_task_bounds
Change-Id: Ibf3c85985fd9372a59ef6a990ce23655ae74076e