Adding accessibility focusable attribute (hidden for now).

1. This attribute specifies whether a view can take accessibility
   focus. It has three values: 1) auto - the system determines
   based on whether the view is actionable and has actionable
   predecessor. Accessibility services can put accessibility focus
   on such a node at will; 2) yes ; this view always takes access
   focus; 3) no - the view cannot takes accessibility focus and
   accessibility services cannot put accessibility focus on it.

Change-Id: I2ebf4e7c75bf6b39e1742b6868b37ccdd4cc7d28