Move ownership of voe::Channel into Audio[Receive|Send]Stream.

* VoEBase contains only stub methods (until downstream code is
  updated).

* voe::Channel and ChannelProxy classes remain, but are now created
  internally to the streams. As a result,
  internal::Audio[Receive|Send]Stream can have a ChannelProxy injected
  for testing.

* Stream classes share Call::module_process_thread_ for their RtpRtcp
  modules, rather than using a separate thread shared only among audio
  streams.

* voe::Channel instances use Call::worker_queue_ for encoding packets,
  rather than having a separate queue for audio (send) streams.

Bug: webrtc:4690
Change-Id: I8059ef224ad13aa0a6ded2cafc52599c7f64d68d
Reviewed-on: https://webrtc-review.googlesource.com/34640
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21578}
diff --git a/call/BUILD.gn b/call/BUILD.gn
index 90460e7..94b0f15 100644
--- a/call/BUILD.gn
+++ b/call/BUILD.gn
@@ -208,6 +208,7 @@
       "../api:libjingle_peerconnection_api",
       "../api:mock_audio_mixer",
       "../api/audio_codecs:builtin_audio_decoder_factory",
+      "../audio:audio",
       "../logging:rtc_event_log_api",
       "../modules/audio_device:mock_audio_device",
       "../modules/audio_mixer",