AGC2 dummy module: fixed gain param, APM integration, audioproc_f adaptation

In preparation of coming CLs that will add an AGC interface to make the
gain controller injectable.

This CL simplifies AGC2 (dummy sub-module of audioproc_f) since it only
implements the fixed digital mode with hard-clipping - i.e., no limiter
is used.
The AGC2 config now includes the fixed gain to apply and audioproc_f
has been adapted accordingly.
Finally, this CL slightly simplifies the AGC2 integration into APM.

This CL is a continuation of https://codereview.webrtc.org/2995043002/

Bug: webrtc:7494
Change-Id: I3d554ea4dc6208928352059feb14987edabf14c7
Reviewed-on: https://webrtc-review.googlesource.com/4661
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20278}

modules/audio_processing/include/audio_processing.h

@@ -340,6 +340,7 @@ class AudioProcessing : public rtc::RefCountInterface {
     // does not yet have the desired behavior.
     struct GainController2 {
       bool enabled = false;
+      float fixed_gain_db = 0.f;
     } gain_controller2;
 
     // Explicit copy assignment implementation to avoid issues with memory