Fix issue #21621920: VI: need mechanism to get current request

Add new APIs to associate a Request with a name, get all active
requests, and get active request by name.

Also add a new Activity.onProvideReferrer() which will allow
applications to propagate referrer information to the assistant
(and other apps they launch) in a consistent way.

Change-Id: I4ef74b5ed07447da9303a74a1bdf42e4966df363