To get the recording duration, we were using the camerabin's position,
which represents the time since it was started, not the time it's been
recording to a file.
We now retrieve the camerabin's filesink position.
Change-Id: I68eeb25d1718666288655d22deea23e25de73b90
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>