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 Åhgren <peah@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22873}

modules/audio_processing/audio_processing_impl_unittest.cc

@@ -74,4 +74,10 @@ TEST(AudioProcessingImplTest, AudioParameterChangeTriggersInit) {
   EXPECT_NOERR(mock.ProcessReverseStream(&frame));
 }
 
+TEST(AudioProcessingImplTest, UpdateCapturePreGainRuntimeSetting) {
+  // TODO(bugs.chromium.org/9138): Implement this test as soon as the pre-gain
+  // sub-module is implemented and it is notified by HandleRuntimeSettings()
+  // when the gain changes.
+}
+
 }  // namespace webrtc