Added scroll and text selection change accessibility events.

1. Added scrolling accessibility event to provicde feedback
   when a view is scrolled.

Note: We need scroll events for ICS since even though we have
      touch exploration the user does not know when something
      is scrollable and not feedback is provided while scrolling.

bug:4902097

2. Added a text selection change event to provide feedback
   for selection changes including cursor movement.

Note: We need the text selection change events for ICS since
      even though the IME supports navigation in text fields
      the user receives no feedback for the current selection/
      cursor position.

bug:4586186

3. Added a scrollable property to both AccessibilityEvent and
   AccessibilityNodeInfo. The info has to describe the source
   in terms of all properties that make sense for accessibility
   purposes and the event has this property (kinda duplicated)
   since clients will aways want to know if the source is
   scrollable to provided clue to the user and we want to avoid
   pulling the info of the source for every accessibility event.

Change-Id: I232d6825da78e6a12d52125f51320217e6fadb11