More work on voice interaction visuals.

There is now a special theme for voice interaction activities
to use, so they can be a panel that is better intergrated with
the rest of the voice interaction experience.  This is still
not completely working, I have some hacks in the demo app to
get it right; I'll fix that in a future change.

Also improve VoiceInteractor to be retained across activity
instances, for things like rotation.

And bump up the number of concurrent broadcasts that are allowed
on non-svelte devices, since they can handle more and this makes
the boot experience better when dispatching BOOT_COMPLETED.

Change-Id: Ie86b5fd09b928da20d645ec2200577dee3e6889d