Fix bug 5237737 - ActionBar should fire an accessibility event when

action mode is turned on/off

Fire AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED when action modes
come and go to give an indication of UI change on the level of a menu
or dialog opening/closing.

Change-Id: Id36c6153b0722b4b6927c8d36503e8ac57c2d2b2