VoEBase is plumbed to optionally take an AudioDecoderFactory, or create
a builtin factory if none is provided.
Retained the CreateChannel interfaces in Channel and ChannelManager
and added variants for injecting an AudioDecoderFactory. The
"old-style" variants call CreateBuiltinAudioDecoderFactory to get a
factory to use.
(Just realized this means each channel uses a separate factory with the
old-style calls. Probably ok.)
BUG=webrtc:5805
Review-Url: https://codereview.webrtc.org/1993783002
Cr-Commit-Position: refs/heads/master@{#12961}
5f7cfa50e5