First step of AudioDecoderFactory injection CLs. AudioDecoderFactories will be shared, and shared_ptr is currently off the table, so this CL changes the current uses of AudioDecoderFactory from std::unique_ptr to rtc::scoped_refptr. BUG=webrtc:5805 Review-Url: https://codereview.webrtc.org/1990803004 Cr-Commit-Position: refs/heads/master@{#12815}