Fix issues with video auto-fullscreen mode.

There were a few issues with the auto-fullscreen mode code which is present in
video calls.
1. If the Incall UI is backgrounded, the auto-fullscreen mode code would still
trigger and put the user into fullscreen mode when they return to the InCall
UI.
2. If the user is already in a video call and the remote party enables their
camera, the auto fullscreen mode code would trigger and enable fullscreen mode.
Ultimately we only want to go into fullscreen mode if we are entering a video
call from a non-video state.
3. It is possible for the call to cease to be a video call by the time the auto
fullscreen code triggers, meaning that an audio only call would go into
fullscreen mode:

Change-Id: I338a9ba4bb8d171752dcb38c16363bf0ded701af
CL: 120864913
Bug: 28093622