media/gpu/v4l2: abstract config store management in V4L2DecodeSurface

Config store is a non-official, Chrome OS-only substitute to request
API, added at a time where no solution existed for stateless codecs.

Support for it is a bit all over the place. Since the official request
API support for stateless codecs is getting closer to being merged, we
want to support this as well.

This CL prepares for the addition of the request API by abstracting the
config store operations and containing them entirely in
V4L2DecodeSurface.

BUG=917279
TEST=VDA successfully completes on Kevin.

Change-Id: I3f0fa212ae5d8f722eab01d1134b2976416fad11
Reviewed-on: https://chromium-review.googlesource.com/c/1388067
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621920}
7 files changed