AEC3: Increasing the transparency during call startup

This CL increases the AEC3 transparency during call
startup and after echo path delay changes in 3 ways:
1. The exit requirements for the initial mode is
made less strict.
2. The requirements for using the linear echo model
are made less strict.
3. The duplicated reverb modelling in the linear mode
removed.


Bug: webrtc:9572,chromium:868329
Change-Id: I79ea0796ed26408e35576bb39eaae4e4848b4f83
Reviewed-on: https://webrtc-review.googlesource.com/90868
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24132}

modules/audio_processing/aec3/aec_state.h

@@ -182,6 +182,8 @@ class AecState {
   const bool use_stationary_properties_;
   const bool enforce_delay_after_realignment_;
   const bool allow_linear_mode_with_diverged_filter_;
+  const bool early_filter_usage_activated_;
+  const bool use_short_initial_state_;
   ErlEstimator erl_estimator_;
   ErleEstimator erle_estimator_;
   size_t capture_block_counter_ = 0;