Remove dependency on android.telecom.Call in VideoCallImpl for testing.

VideoCallImpl had a depedency on android.telecom.Call, which was used
to get the current video state of a call when the user issues a
session modify request (we need to know what the video state was before
the request was sent).  This proved problematic for unit tests, as
android.telecom.Call is a final class and cannot be mocked.

These changes assume the VideoCallImpl will instead have a video state
property, which is updated by the Call whenever it changes.  This
removes the dependency on the Call, and makes it possible to unit test the
API.

Change-Id: Ie67255d68b23e32aa177b30ac6439632fad5cc27