base

The logic for not populating text to some  accessibility events is scattered.

1. Some accessibility evenents should not and were not dispatched for
   text population but there was no centralized location for enforcing
   this - rather the system was firing them in a specific way or there
   were conditions in a few places enforcing that. Now this is centralized
   and clean.

2. Updated the documentation with some new event types the were lacking.

3. Explicitly stated in the documentaition which events are dispatched to
   the sub-tree of the source for text populatation.

bug:5394527

Change-Id: I86e383807d777019ac98b970c7d9d02a2f7afac6