`AgcManagerDirectTestHelper` simplified and API contract compliant

Main changes:
- `AgcManagerDirectTestHelper::FirstProcess()` replaced by
  `CallAgcSequence()`, which is API contract compliant
- `ExpectCheckVolumeAndReset()`, `SetVolumeAndProcess()` and
  `ExpectInitialize() `removed
- TODOs added for the next batch of improvements
- `AgcManagerDirectTestHelper::mock_agc` now using `NiceMock`
- `AgcManagerDirect::(AnalyzePre)Process()` now receives a
  const ref
- `AnalyzePreProcess(const float* const*,size_t )` removed

Bug: webrtc:7494
Change-Id: Ie5bbaa590586dd806b30494fb00ca9c742c241e2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/273490
Reviewed-by: Hanna Silen <silen@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38056}
diff --git a/modules/audio_processing/audio_processing_impl.cc b/modules/audio_processing/audio_processing_impl.cc
index 0857b28..81d9660 100644
--- a/modules/audio_processing/audio_processing_impl.cc
+++ b/modules/audio_processing/audio_processing_impl.cc
@@ -1161,7 +1161,7 @@
   }
 
   if (submodules_.agc_manager) {
-    submodules_.agc_manager->AnalyzePreProcess(capture_buffer);
+    submodules_.agc_manager->AnalyzePreProcess(*capture_buffer);
   }
 
   if (submodule_states_.CaptureMultiBandSubModulesActive() &&
@@ -1234,7 +1234,7 @@
   }
 
   if (submodules_.agc_manager) {
-    submodules_.agc_manager->Process(capture_buffer);
+    submodules_.agc_manager->Process(*capture_buffer);
 
     absl::optional<int> new_digital_gain =
         submodules_.agc_manager->GetDigitalComressionGain();