After we switched to service:media, the directory structure of chromecast/media seems outdated. It is my understanding that most (all?) of the code under chromecast/media is used for cma pipeline. So we should get rid of cma directory and merge the cma subdirectories with top-level chromecast/media.

I also noticed that libcast_media targets are in base/BUILD.gn, which seems wrong. I think they should in backend/BUILD.gn.

Tentatively assigning to slan@ since he is working on media service.