Improve code quality in modules/audio_processing/agc/

- Switch from ptr+size to rtc::ArrayView
- Remove `AgcManagerDirect::sample_rate_hz_` since it's always 16 kHz
- Stop passing nullptr in agc_manager_direct_unittest.cc when
  `AgcManagerDirect::Process()` is called
- Allow to correctly run the tests added in the child CL (see [1])

[1] https://webrtc-review.googlesource.com/c/src/+/250141

Bug: webrtc:7494
Change-Id: I0292d7038d6510ca7c58af32b6003a1e4b121b00
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250541
Reviewed-by: Hanna Silen <silen@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35910}

modules/audio_processing/audio_processing_impl.h

@@ -18,6 +18,7 @@
 #include <string>
 #include <vector>
 
+#include "api/array_view.h"
 #include "api/function_view.h"
 #include "modules/audio_processing/aec3/echo_canceller3.h"
 #include "modules/audio_processing/agc/agc_manager_direct.h"