Send runtime settings to the Audio Processing Module (APM)

This CL includes the following changes:
- APM runtime setting (ID + float payload) and unit tests
- Swap queue of APM runtime settings used in AudioProcessingImpl
- runtime settings handler that forwards the settings to APM
  sub-modules when a message is retrieved from the queue
- Unit test placeholder to check that the pre-gain update message
  is correctly delivered

Bug: webrtc:9138
Change-Id: Id22704af15fde2b87a4431f5ce64ad1aeafc5280
Reviewed-on: https://webrtc-review.googlesource.com/69320
Reviewed-by: Per Ã…hgren <peah@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22873}
diff --git a/modules/audio_processing/include/mock_audio_processing.h b/modules/audio_processing/include/mock_audio_processing.h
index 96a04ef..2d49388 100644
--- a/modules/audio_processing/include/mock_audio_processing.h
+++ b/modules/audio_processing/include/mock_audio_processing.h
@@ -167,6 +167,7 @@
   MOCK_CONST_METHOD0(num_output_channels, size_t());
   MOCK_CONST_METHOD0(num_reverse_channels, size_t());
   MOCK_METHOD1(set_output_will_be_muted, void(bool muted));
+  MOCK_METHOD1(SetRuntimeSetting, void(RuntimeSetting setting));
   MOCK_METHOD1(ProcessStream, int(AudioFrame* frame));
   MOCK_METHOD7(ProcessStream, int(const float* const* src,
                                   size_t samples_per_channel,