[AT-SPI] Require ControlAccessible for Control

[AT-SPI] Require ControlAccessible for Control

This change narrows down the type used in GetAccessibilityObject() and
SetAccessibilityConstructor() from Accessible to ControlAccessible. It
has always been the case that a Control is represented by either a
ControlAccessible or a more derived type, and this fact is now
reaffirmed in the API.

Change-Id: Iae5f87845d951ac9affe2eb8589ec4aa3ea274e6