More API changes.

Start moving Assist* stuff to android.app.assist.

Clean up some more of the VoiceInteractionSession APIs.

Clearly document that finish() is not the same as hide(),
always call hide() instead, and fix the finish() path to
also always do a hide to make sure everything is cleaned
up correctly.

Change-Id: I962d4069fcb34fe89547a95d395ae1b9fa3b4148