screen-share: Start screen sharing even if no pointer is found

With commit e825fe38, we no longer display the pointer if no movement is
detected, which will cause screen share to fail to start if that is the
case. There could be also legitimate cases where there's no pointer, so
let's allow screen share to function in those cases as well. Makes uses
of previous helper methods to find a proper output to share in case we
don't have an pointer.

Re-uses the shell utils functions.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>