Add support for external encoders in ACM

Also introduce tests using external (mock) encoders, both for
CodecOwner and for AudioCodingModule.

Support for external decoders is still missing.

COAUTHOR=henrik.lundin@webrtc.org
BUG=4474
R=jmarusic@webrtc.org, minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49939004

Cr-Commit-Position: refs/heads/master@{#9206}
12 files changed