LibMedia: Add a stub implementation of FFmpegVideoDecoder for Android

We don't have ffmpeg available on Android, so provide a dummy
FFmpegVideoDecoder and disable linking against ffmpeg for Android.

Userland/Libraries/LibMedia/FFmpeg/FFmpegVideoDecoderStub.cpp

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2024, Alex Studer <alex@studer.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/System.h>
+#include <LibMedia/VideoFrame.h>
+
+#include "FFmpegForward.h"
+#include "FFmpegVideoDecoder.h"
+
+namespace Media::FFmpeg {
+
+DecoderErrorOr<NonnullOwnPtr<FFmpegVideoDecoder>> FFmpegVideoDecoder::try_create(CodecID codec_id, ReadonlyBytes codec_initialization_data)
+{
+    (void)codec_id;
+    (void)codec_initialization_data;
+    return DecoderError::format(DecoderErrorCategory::NotImplemented, "FFmpeg not available on this platform");
+}
+
+FFmpegVideoDecoder::FFmpegVideoDecoder(AVCodecContext* codec_context, AVPacket* packet, AVFrame* frame)
+    : m_codec_context(codec_context)
+    , m_packet(packet)
+    , m_frame(frame)
+{
+}
+
+FFmpegVideoDecoder::~FFmpegVideoDecoder()
+{
+}
+
+DecoderErrorOr<void> FFmpegVideoDecoder::receive_sample(Duration timestamp, ReadonlyBytes sample)
+{
+    (void)timestamp;
+    (void)sample;
+    return DecoderError::format(DecoderErrorCategory::NotImplemented, "FFmpeg not available on this platform");
+}
+
+DecoderErrorOr<NonnullOwnPtr<VideoFrame>> FFmpegVideoDecoder::get_decoded_frame()
+{
+    return DecoderError::format(DecoderErrorCategory::NotImplemented, "FFmpeg not available on this platform");
+}
+
+void FFmpegVideoDecoder::flush()
+{
+}
+
+}