Introduce new AssistData class that contains all data
the framework automatically generates for assist. Currently
populated with a very simple tree structure representing
the app's view hierarchy.
Reworked how we populate the class name for accessibility
info, so this is provided through a new method call on View
that subclasses can override. This method is also used
to populate the class name in AssistData.
Change-Id: Ibd0acdc8354727d4291473283b5e4b70894905dc
a7bb6fbeab