Camera: Fix hotplug

According to the camera_module_t::get_number_of_cameras() description
in camera_common.h, it should only return the number of "fixed"
cameras, i.e. cameras, facing to the front and to the back. Any
cameras, with the facing value of "external," i.e. externally connected
cameras, should not be reported by it, instead they should later be
reported, using the
camera_module_callbacks_t::camera_device_status_change() callback.
However, this doesn't work. When a camera is reported this way, it is
ignored as unknown. Fix this by adding a new camera, when its status
change is reported.

Change-Id: I18874ed005f477e32fb723c00c16b67a81cde6ce
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@intel.com>