First quick implementation of auto assist data.

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