Implements fdk_aac decoder (#4764)

* audio_core: dsp_hle: implements fdk_aac decoder

* audio_core: dsp_hle: clean up and add comments

* audio_core: dsp_hle: move fdk include to cpp file

* audio_core: dsp_hle: detects broken fdk_aac...

... and refuses to initialize if that's the case

* audio_core: dsp_hle: fdk_aac: address comments...

... and rebase commits

* fdk_decoder: move fdk header to cpp file

src/audio_core/hle/fdk_decoder.h

@@ -0,0 +1,23 @@
+// Copyright 2019 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "audio_core/hle/decoder.h"
+
+namespace AudioCore::HLE {
+
+class FDKDecoder final : public DecoderBase {
+public:
+    explicit FDKDecoder(Memory::MemorySystem& memory);
+    ~FDKDecoder() override;
+    std::optional<BinaryResponse> ProcessRequest(const BinaryRequest& request) override;
+    bool IsValid() const override;
+
+private:
+    class Impl;
+    std::unique_ptr<Impl> impl;
+};
+
+} // namespace AudioCore::HLE