Cleanup after moving test/fake_audio_device to modules/audio_device/include/test_audio_device. Hide implementation of test audio device module in the anonymous namespace. Bug: webrtc:8946 Change-Id: I2d49c3ec5d43eeb5f155d38de95f69ed3c537805 Reviewed-on: https://webrtc-review.googlesource.com/61426 Commit-Queue: Artem Titov <titovartem@webrtc.org> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org> Cr-Commit-Position: refs/heads/master@{#22401}