Fixing accessibility scrolling events generated by PagedView:

1. Not generating scroll events from snapToPage(). It already gets
   generated from computeScrollHelper().

2. Not setting action because doing so is not mentioned here:
   http://developer.android.com/reference/android/view/accessibility/AccessibilityEvent.html.

3. Not generating scroll event when the page stays same (before it was
   generated, say, when we simply returned from the AllApps view to
   Workspace).

4. From/To index is not the old and new page numbers; they are indices
   of the first and last item; in our case, the item is the page, and
   both FromIndex and ToIndex should be set to this page number.

Bug: 18761184
Change-Id: I3dadf816c3d45b8bd42a13930344874584467499