When a change in made from video from a camera to desktop streaming (or vice
versa) it is necessary to explicitly change the video device of the Call.
Otherwise, the Call's default device doesn't change. In OpSetVideoTelephony
this was done any time video is enabled/disabled, and this resulted in
unnecessary changes of the MediaStream device from one wrapper of the camera
device to another. This in turn sometimes led (for an unknown reason) to the
camera light being turned on when it is not used, and in some cases to a JVM
crash.

This commit resets the video device of a Call when its use case changes. This
avoids doing the same in OpSetVideoTelephony when video is enabled/disabled,
and assures that the device is changed with every change of the use case, and
only when the use case changes.
Fix an NPE if we try to generate conference-info for a CallPeer without a Call