Encapsulate the ViewRootImpl's handler.

This change makes it much easier to make sense of the messages that
get posted to the ViewRootImpl's handler by encapsulating their point
of dispatch within the ViewRootImpl itself.

As part of this change, the View.AttachInfo now carries a reference
to the ViewRootImpl itself, which simplifies some code that used
to try to find the ViewRootImpl by getting the root view's parent.

In principle, it might have been nice to hide the ViewRootImpl from
the View hierarchy but in practice the two were coupled in many ways.

Change-Id: I51ebccdf5f8c8c505cd6f17cdf594174d041dc54