Moves the unified AdaptationReason to the api/ folder. Bug: webrtc:11392 Change-Id: I28782e82ef6cc3ca3b061f65b0bbdc3766df1f9c Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/172583 Commit-Queue: Evan Shrubsole <eshr@google.com> Reviewed-by: Erik Språng <sprang@webrtc.org> Reviewed-by: Niels Moller <nisse@webrtc.org> Reviewed-by: Henrik Boström <hbos@webrtc.org> Cr-Commit-Position: refs/heads/master@{#31084}