Updating the behaviour of accessibility text iterators.

1. Iterators were skipping content on reversing direction.

2. The cursor was positioned at the beginning of the next text segment
   when moving forward and at end of the previous text segment when moving
   backwards. This is incorrect and now the cursor is positioned at the
   end of the segment when moving forward and at the beginning when moving
   backward.

3. The cursor position was not properly set when reaching the end/start
   of the text.

4. The iterators were reporting strictly the next/previous segment even
   if the cursor is within such a segment. Thus, when traversing some
   content may be skipped. Now moving forward moves the selection to
   the next segment end and the start position is either the old index
   if it was within a segment or the start of the segment. Same in
   reverse.

bug:6575099

Change-Id: Ib48a649cec53910339baf831a75e26440be6e576