Move FuzzAudioEncoder to a separate target to make it available for
other encoders.
Bug: webrtc:11388
Change-Id: I8b9a0f810791880eedb129b55eb33f154790e48f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169364
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30634}
