[arc] Use SharedImageFormat in ArcScreenCaptureSession

As part of BufferFormat to SharedImageFormat conversions, update
ArcScreenCaptureSession to use SharedImageFormat. Also update the
corresponding mojom file.

Bug: 356649879
Change-Id: Ib38159c9ac2d0d1cbc21220269105f8a550c1980
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6911847
Commit-Queue: Saifuddin Hitawala <hitawala@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1510347}