Rework some of the voice interaction APIs.

On the app side, requests are now composed by subclassing
from various types of Request objects.

On the service side, starting a voice interaction session
involves starting another service that will then manage the
session.  This leads the service design much more to what
we want, where the long-running main service is very tiny
and all the heavy-weight transient session work is elsewhere
in another process.

Change-Id: I46c074c6fe27b6c1cf2583c6d216aed1de2f1143