[TreeList] Select-all checkbox does not update its state when selection is set programmatically

[zstoyanova9]

Description

When programmatically setting the selected TreeList rows, the select-all checkbox does not update its state accordingly. This behavior can also be observed in the following demos:
https://www.telerik.com/kendo-angular-ui/components/treelist/selection/persisting#persisting-row-selection-based-on-a-specific-field
https://www.telerik.com/kendo-angular-ui/components/treelist/selection/persisting#persisting-row-selection-based-on-computed-values

Steps To Reproduce

1. Configure a TreeList with checkbox selection and a select-all checkbox enabled.
2. Set some selected rows for the TreeList programmatically (e.g. through the selectedItems property).
3. Observe how the select-all checkbox does not have an indeterminate state applied even though there are some selected items.

StackBlitz example for demonstration - https://stackblitz.com/edit/angular-hrcsdbkf

Actual Behavior

The select-all checkbox remains empty and does not apply a correct state when the selection is set programmatically.

Expected Behavior

The select-all checkbox should update its state according to the currently selected items despite being programmatically set.