Refactor media constants to inline constexpr absl::string_view

This CL converts most string constants in media/base/media_constants.h
to inline constexpr absl::string_view, moving their definitions to the
header file. Constants marked with RTC_EXPORT are preserved as
extern const char[] for ABI compatibility.

Infrastructure updates:
- Updated CodecParameterMap to use a transparent comparator (std::less<>)
  allowing lookups with string_view.
- Updated Codec, SdpVideoFormat, and SdpAudioFormat to better handle
  absl::string_view in constructors and parameter methods.
- systemically updated downstream call sites to use SetParam or explicit
  std::string conversions where necessary.

Bug: webrtc:42223790
Change-Id: I017fd4e6c85b072a8d31aa0aec09cb1264464627
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/467720
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48098}