Factored out the Accessibility vertical initialization.

1. Accessibility events were filled with data in
   dispatchPopulateAccessibilityEvent and
   onPopulateAccessibilityEvent. These events have
   two axis of population 1) up the class
   hierarchy to populate information for the event
   source; 2) down the view hierarchy to populated
   all the text contained in the source including
   its descendants. These two axis of population
   were done in on population pass now the populating
   the source properties happens in initializeAccessiblityEvent
   and the text in onPopulateAccessibilityEvent which
   is called from dispatchPopulateAccessiblityEvent.

2. Removed the string description from events fired from
   CompoundButton since the event has isChecked()
   property and it is responsibility of the clients
   to decide what utterrance to use and if to use such
   for announcing the checked state.

Change-Id: I5d7f75cf8a87a7a4b3bb7b311e8e642ec9a0faa5