No real encoder implements the correct API yet, so we're just testing
dummies.
BUG=webrtc:7823
Review-Url: https://codereview.webrtc.org/2935643002
Cr-Commit-Position: refs/heads/master@{#18637}
Plumbed AudioEncoderFactory up into CreatePeerConnectionFactory.
BUG=webrtc:5806
Review-Url: https://codereview.webrtc.org/2799033006
Cr-Commit-Position: refs/heads/master@{#17977}