[INTERNAL] m.ComboBox: update "aria-activedescendant" on keystroke

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

Change-Id: Ib063e52e181fde1ab62cd1ae8d1ec2d55ccce606
Merge "[INTERNAL] m.ComboBox: update "aria-activedescendant" on keystroke"