Add SetAudioPlayout and SetAudioRecording methods to the PeerConnection API (II)

Second attempt to land https://webrtc-review.googlesource.com/c/src/+/16180

Now removes voice_engine dependency from peerconnection and fixes a minor
const issue in NullAudioPoller.

TBR=solenberg

Bug: webrtc:7313
Change-Id: Ibfddbdc76118581e4a4dc64575203f84c1659e5c
Reviewed-on: https://webrtc-review.googlesource.com/17784
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Henrik Andreassson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20526}
diff --git a/audio/BUILD.gn b/audio/BUILD.gn
index a9ca0d5..80545ca 100644
--- a/audio/BUILD.gn
+++ b/audio/BUILD.gn
@@ -23,6 +23,8 @@
     "audio_transport_proxy.cc",
     "audio_transport_proxy.h",
     "conversion.h",
+    "null_audio_poller.cc",
+    "null_audio_poller.h",
     "scoped_voe_interface.h",
     "time_interval.cc",
     "time_interval.h",
@@ -52,6 +54,7 @@
     "../modules/pacing:pacing",
     "../modules/remote_bitrate_estimator:remote_bitrate_estimator",
     "../modules/rtp_rtcp:rtp_rtcp",
+    "../rtc_base:rtc_base",
     "../rtc_base:rtc_base_approved",
     "../rtc_base:rtc_task_queue",
     "../system_wrappers",