Add one-stop-shop for built-in AEC toggling in APM

This does not change what AEC functionality is available.
However, a client that only uses this interface - and not the submodule
pointer accessors - gets simpler code, and is guaranteed not to run any
two AECs in tandem.

The submodule interface EchoControlMobile is being deprecated in
https://webrtc-review.googlesource.com/c/src/+/89392

Bug: webrtc:9535
Change-Id: Id9326074e566be6d8768010fc421c457beff402c
Reviewed-on: https://webrtc-review.googlesource.com/89386
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24066}

modules/audio_processing/audio_processing_impl.h

@@ -393,6 +393,8 @@ class AudioProcessingImpl : public AudioProcessing {
     int prev_analog_mic_level;
   } capture_ RTC_GUARDED_BY(crit_capture_);
 
+  // This struct requires EITHER crit_capture_ OR crit_render_ to read, and
+  // requires both locks to write.
   struct ApmCaptureNonLockedState {
     ApmCaptureNonLockedState(bool intelligibility_enabled)
         : capture_processing_format(kSampleRate16kHz),