Rework assist to walk down the view hierarchy.

Instead of collecting all of the data directly in AssistStructure,
we now have a dispatch mechanism down the hierarchy to do so.
While doing this, also added the ability to automatically collect
assist data from AccessibilityNodeProviders attached to views
(so now we see all of the data in for example Calendar).

This is a first step needed towards being able to asynchronously
populate assist data.

Change-Id: I59ee1ea104ca8207bad8df7a38195d93da1adea7