audio_processing/agc: Adds config to set minimum microphone volume at startup

The AGC is currently bumping up the mic volume to 33% at startup if it is below that level. This is to avoid getting stuck in a poor state from which the AGC can not move, simply a too low input audio level. For some users, 33% is instead too loud.

This CL gives the user the possibility to set that level at create time.
- Extends the Config ExperimentalAgc with a startup_mic_volume for the user to set if desired. Note that the bump up does not apply to the legacy AGC and the "regular" AGC is controlled by ExperimentalAgc.
- Without any actions, the same default value as previously is used.
- In addition I removed a return value from InitializeExperimentalAgc() and InitializeTransient()

This has been tested by building Chromium on Mac and verify through apprtc that
1) startup_mic_volume = 128 bumps up to 50%.
2) startup_mic_volume = 500 (out of range) bumps up to 100%.
3) startup_mic_volume = 0 bumps up to 4%, the AGC min level.

BUG=4529
TESTED=locally
R=andrew@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43109004

Cr-Commit-Position: refs/heads/master@{#9004}

webrtc/modules/audio_processing/audio_processing_impl.cc

@@ -177,6 +177,7 @@ AudioProcessingImpl::AudioProcessingImpl(const Config& config,
 #else
       use_new_agc_(config.Get<ExperimentalAgc>().enabled),
 #endif
+      agc_startup_min_volume_(config.Get<ExperimentalAgc>().startup_min_volume),
       transient_suppressor_enabled_(config.Get<ExperimentalNs>().enabled),
       beamformer_enabled_(config.Get<Beamforming>().enabled),
       beamformer_(beamformer),
@@ -285,15 +286,9 @@ int AudioProcessingImpl::InitializeLocked() {
     }
   }
 
-  int err = InitializeExperimentalAgc();
-  if (err != kNoError) {
-    return err;
-  }
+  InitializeExperimentalAgc();
 
-  err = InitializeTransient();
-  if (err != kNoError) {
-    return err;
-  }
+  InitializeTransient();
 
   InitializeBeamformer();
 
@@ -959,19 +954,19 @@ bool AudioProcessingImpl::analysis_needed(bool is_data_processed) const {
   return false;
 }
 
-int AudioProcessingImpl::InitializeExperimentalAgc() {
+void AudioProcessingImpl::InitializeExperimentalAgc() {
   if (use_new_agc_) {
     if (!agc_manager_.get()) {
-      agc_manager_.reset(
-          new AgcManagerDirect(gain_control_, gain_control_for_new_agc_.get()));
+      agc_manager_.reset(new AgcManagerDirect(gain_control_,
+                                              gain_control_for_new_agc_.get(),
+                                              agc_startup_min_volume_));
     }
     agc_manager_->Initialize();
     agc_manager_->SetCaptureMuted(output_will_be_muted_);
   }
-  return kNoError;
 }
 
-int AudioProcessingImpl::InitializeTransient() {
+void AudioProcessingImpl::InitializeTransient() {
   if (transient_suppressor_enabled_) {
     if (!transient_suppressor_.get()) {
       transient_suppressor_.reset(new TransientSuppressor());
@@ -980,7 +975,6 @@ int AudioProcessingImpl::InitializeTransient() {
                                       split_rate_,
                                       fwd_out_format_.num_channels());
   }
-  return kNoError;
 }
 
 void AudioProcessingImpl::InitializeBeamformer() {