[CaptureMode]: Gate FakeCameraDevice SCANOUT usage on SI support

FakeCameraDevice adds SCANOUT SharedImage usage in contexts where
overlays are not actually supported (e.g., ChromeOS-on-Linux in
testing). As a result, GLTextureImageBacking needs to support SCANOUT
usage even though it is not actually capable of producing overlays.

This CL guards CameraVideoFrameHandler's addition of SCANOUT usage by
the relevant SharedImage support for scanout being present.

Bug: 330865436
Change-Id: I2f73bc617ff72ea6abbd5cb94b56107e2efde046
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5993430
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378842}