Capture semantic Notification.Builder data in extras.

This is an important step toward being able to reason about
Notifications after they've been built and sent to the
NoMan; right now most of that information is baked into the
contentView and bigContentView before being thrown away.

TODO: more stuff. Right now we're just grabbing the basics:
title, text, subtext, and small icon res ID.

Change-Id: Ief725bb38d160d340b0e3c46f83bc02d5a283d13