Add system layer for voice interaction services.

New window layer that voice interaction service windows
go in to.  Includes a new voice-specific content rectangle
that voice activities are placed in to.

Add specific animations for this layer, sliding down from
the top (though this can be customized by the voice interaction
service).

Also add the concept of activities running for voice interaction
services for purposes of adjusting the animation used for them,
again sliding from the top, but not (yet?) customizable by the
voice interaction service.

Change-Id: Ic9e0e8c843c2e2972d6abb4087dce0019326155d