Further work on voice interaction services.

This makes VoiceInteractionSession a more first-class
concept.  Now the flow is that a VoiceInteractionService
calls startSession() when it wants to begin a session.
This will result in a new VoiceInteractionSession via the
VoiceInteractionSessionService containing it, and the
session at that point an decide what to do.  It can now
show UI, and it is what has access to the startVoiceActivity
API.

Change-Id: Ie2b85b3020ef1206d3f44b335b128d064e8f9935