platform_decoding: Split log streams.

Previously, stdout, stderr, and decoding/validation errors were dumped
together to stdout. Dump them to their respective streams instead.

BUG=b:350824289
TEST=emerge-rex and deploy, tast run --var=expectations.disable=true video.PlatformDecoding.ffmpeg_vaapi_hevc_files_from_bugs_241731425

Change-Id: Ieffb8149517d5751e8a4daf4c2ba3f0aa6a31deb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/graphics/+/5685848
Commit-Queue: Miguel Casas <mcasas@google.com>
Reviewed-by: Miguel Casas <mcasas@google.com>
Auto-Submit: Jao-ke Chin-Lee <jchinlee@google.com>
Reviewed-by: Miguel Casas-Sanchez <mcasas@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@google.com>
Tested-by: Jao-ke Chin-Lee <jchinlee@google.com>
2 files changed