Avoid unnecessary copying in media engine and base

Code in media/engine and media/base copies codec lists, codecs and other
objects that it only reads, on paths such as codec negotiation,
SetParameters and GetStats, often once per element inside find_if
lambdas. Some by-value parameters and locals are also copied at their
last use instead of being moved.

- Use const references for read-only values: VideoCodecSettings vectors,
  the elements visited by find_if lambdas, StreamParams and the audio
  track id. Compare codec names as absl::string_view instead of
  constructing std::string temporaries.
- Move by-value parameters and locals at their last use, such as
  send_codec in the RTP parameter checks and the frame encryptor,
  decryptor and transformer scoped_refptrs. Use emplace instead of
  insert for decoder_map to avoid constructing a temporary pair.

Bug: webrtc:374845009
Change-Id: I254fb9bd79340929a233544a4d626e636bc44530
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/498960
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48811}