Text in accessibility events not consistent

bug:2513822

Text added to accessibility events is truncated to max
length. However, the fromIndex and toIndex properties
are relative to the text before being truncated, thus
potentially our of bound. Removed the max length
limitation because test very rarely is longer that
500 characters and in the cases in which text is longer
than 500 character it will not be a real problem to
pass a bit more data through an IPC (very rarely).

Change-Id: Ie70ac630dfeb56d4f59079abb2f46d07582796b6