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ToT V4L2VideoDecoder::Initialize() tries to Open() |device_| with both
stateless and stateful formats -- which is fine, but log printouts are
confusing, as they read oftentimes like:

  [v4l2_video_decoder.cc(166)] Initialize(): Found V4L2 device capable of stateless decoding for S264
  ...
  [v4l2_video_decoder.cc(175)] Initialize(): Failed to open device for profile: 3 fourcc: H264

which seems to imply that Initialize() failed, when in reality it
worked. Moreover, ToT code checks for |input_format_fourcc_stateful|
and |input_format_fourcc_stateless| values and has an else() path that
is never reached (l.218 LHS).

This CL refactors that code to make evident that the stateful API takes
precedence if present and working, sprinkles constexpr names, and takes
the chance to improve some loggings using e.g. FourccToString() and
GetProfileName().

No new functionality intended.

Bug: b/170870476
Change-Id: I6ad00c1ce4d9b8cf2588d7a2751484a743f4225f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616881Reviewed-by: Fritz Koenig <frkoenig@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842210}