recipes-demo: camera-gstreamer: Updates to access cameras via pipewire

- camera-gstreamer app can now work with cameras via pipewire(default), V4L2 API
  is also intact.
- Pipeline fallsback to a still image if the cameras are not available, so that
  users understand the reason for not capturing the cameras.

Bug-AGL: SPEC-4881
Change-Id: Iae5a1ce3bf27301996915f705af413acada1b8a3
Signed-off-by: Ashok Sidipotu <ashok.sidipotu@collabora.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29382
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>