Avoid some unnecessary string and container copies

Several functions copy strings and vectors that they only read or that
are about to go out of scope.

- Move by-value parameters into their destination: the SSRC vector in
  TransportFeedbackDemuxer::RegisterStreamFeedbackObserver, the key and
  value in MediaConstraints::Constraint, which now takes them by value.
- Take absl::string_view for read-only strings in CreateJavaIceCandidate
  and in frame_analyzer's JoinFilename, which now builds the path with
  StringBuilder.

MediaConstraints::Constraint in sdk/media_constraints.h is public API.
Taking the strings by value is source compatible for callers.

Bug: webrtc:374845009
Change-Id: Idff385530c0f7800d6bd90bdb4311a43001cf069
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/498541
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48839}