Only adapt AGC when the desired signal is present

Take the 50% quantile of the mask and compare it to certain threshold to determine if the desired signal is present. A hold is applied to avoid fast switching between states.
is_signal_present_ has been plotted and looks as expected. The AGC adaptation sounds promising, specially for the cases when the speaker fades in and out from the beam direction.

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/28329005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@8078 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_processing/audio_processing_impl.h

@@ -86,6 +86,8 @@ class AudioFormat : public AudioRate {
 class AudioProcessingImpl : public AudioProcessing {
  public:
   explicit AudioProcessingImpl(const Config& config);
+  // Only for testing.
+  AudioProcessingImpl(const Config& config, Beamformer* beamformer);
   virtual ~AudioProcessingImpl();
 
   // AudioProcessing methods.