AEC3: Adding another config parameter and matching json reader with config

This CL:
-Adds another config parameter that controls the duration of the initial
state.
-Adds reading of that parameter in audioproc_f from the json settings file.
-Adds missing reading of another parameter in audioproc_f from the json
settings file.

Bug: webrtc:8671
Change-Id: Ie6164c360492de5e6b0ade8838bbabe214560b5e
Reviewed-on: https://webrtc-review.googlesource.com/94621
Reviewed-by: Jesus de Vicente Pena <devicentepena@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24360}

api/audio/echo_canceller3_config.h

@@ -53,6 +53,7 @@ struct EchoCanceller3Config {
     ShadowConfiguration shadow_initial = {12, 0.9f, 20075344.f};
 
     size_t config_change_duration_blocks = 250;
+    float initial_state_seconds = 2.5f;
   } filter;
 
   struct Erle {