[INTERNAL] m.ComboBox: sets "aria-activedescendant" when visible

- the "aria-activedescendant" attribute is set when the currently
active descendant (the selected item) is visible and in view

Change-Id: If957f341a9d985ca8ea6d8d6d3e9456f2625cfc7
Merge "[INTERNAL] m.ComboBox: sets "aria-activedescendant" when visible"