Add unused AEC toggling config to API

This will be the one way of toggling AEC. The EchoControlMobile and
EchoCancellation interfaces will be removed.

The settings introduced here are not used yet, to allow for smooth
downstream fixes.

Bug: webrtc:9535
Change-Id: I3b1a524a0ab7daf63419d7e5ed47417b9282dbf6
Reviewed-on: https://webrtc-review.googlesource.com/90864
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24129}

modules/audio_processing/include/audio_processing.h

@@ -252,6 +252,12 @@ class AudioProcessing : public rtc::RefCountInterface {
   // by changing the default values in the AudioProcessing::Config struct.
   // The config is applied by passing the struct to the ApplyConfig method.
   struct Config {
+    // TODO(bugs.webrtc.org/9535): Currently unused. Use this to determine AEC.
+    struct EchoCanceller {
+      bool enabled = false;
+      bool mobile_mode = false;
+    } echo_canceller;
+
     struct ResidualEchoDetector {
       bool enabled = true;
     } residual_echo_detector;