General AEC3 transparency improvements

This CL adds some general AEC3 transparency improvements.

Specifically:
-A minimum for how the nearend is masking echo is added.
-A temporal smoothing constant is increased to increase the transparency.
-Parameters are surfaced to the parameter config struct.

Bug: webrtc:8360
Change-Id: I2a4881eb40f4fab53ad740c4001925f0af86bbec
Reviewed-on: https://webrtc-review.googlesource.com/7605
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20200}

modules/audio_processing/include/audio_processing.h

@@ -290,6 +290,8 @@ class AudioProcessing : public rtc::RefCountInterface {
           float m2 = 0.001f;
           float m3 = 0.02f;  // Do not change.
           float m4 = 0.3f;
+          float m5 = 0.3f;
+          float m6 = 0.0001f;
         } gain_mask;
 
         struct EchoAudibility {