[audio_core] Move to intrusive containers

Moving from std::shared_ptr and std::set, etc. to fbl's intrusive containers
had been planned here for quite awhile as evidenced by johngro@'s TODOs;
this also brings us in line with the style guidelines as fbl::RefPtr is
half the size of std::shared_ptr.

TEST: audio_fidl_tests
Change-Id: Id9734cb8a5099f061a43b017b42a9224bc03fffa