AcmReceiver: index decoders by payload type instead of ACM codec ID

Change internal indexing of registered decoders. It makes sense because payload type is unique, while ACM codec ID may not be. This is a step towards allowing for addition of external decoders.

R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44869004

Cr-Commit-Position: refs/heads/master@{#8867}
4 files changed