APM: Add build flag to allow building WebRTC without APM

This CL adds a build flag to allow building the non-test parts
of WebRTC without the audio processing module.
The CL also ensures that the WebRTC code correctly handles
the case when no APM is available.

Bug: webrtc:5298
Change-Id: I5c8b5d1f7115e5cce2af4c2b5ff701fa1c54e49e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171509
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31133}

modules/audio_processing/audio_processing_impl_locking_unittest.cc

@@ -14,6 +14,7 @@
 
 #include "api/array_view.h"
 #include "modules/audio_processing/audio_processing_impl.h"
+#include "modules/audio_processing/test/audio_processing_builder_for_testing.h"
 #include "modules/audio_processing/test/test_utils.h"
 #include "rtc_base/critical_section.h"
 #include "rtc_base/event.h"
@@ -496,7 +497,7 @@ AudioProcessingImplLockTest::AudioProcessingImplLockTest()
                     this,
                     "stats",
                     rtc::kNormalPriority),
-      apm_(AudioProcessingBuilder().Create()),
+      apm_(AudioProcessingBuilderForTesting().Create()),
       render_thread_state_(kMaxFrameSize,
                            &rand_gen_,
                            &render_call_event_,