Scroll accessibility events should not populate text.

Scroll events are used to report position change and should not
contain the text content of the view that fires them because it
is usiually a containter for many other views and the text will
be long and not informative for accessibility purposes. Also
such evens are fired relatively frequently. If a client wants
to fetch some textual content for a scroll event he can use
the interrogation APIs.

bug:5352059

Change-Id: I43e02aca895c8ab16ba82ebe1cee3aea8ce7711a