flvdemux: Use aac codec-data to adjust channels if needed

Flv does not support various channels in AAC stream format, for example
flvdemux detect an audio channels of 2(stereo) when the AAC really is 1(mono).

https://bugzilla.gnome.org/show_bug.cgi?id=797275

gst/flv/gstflvdemux.c

@@ -723,6 +723,7 @@ gst_flv_demux_audio_negotiate (GstFlvDemux * demux, guint32 codec_tag,
   GstCaps *caps = NULL, *old_caps;
   gboolean ret = FALSE;
   guint adjusted_rate = rate;
+  guint adjusted_channels = channels;
   GstEvent *event;
   gchar *stream_id;
 
@@ -784,6 +785,16 @@ gst_flv_demux_audio_negotiate (GstFlvDemux * demux, guint32 codec_tag,
         } else {
           adjusted_rate = rate;
         }
+
+        adjusted_channels =
+            gst_codec_utils_aac_get_channels (map.data, map.size);
+
+        if (adjusted_channels && (channels != adjusted_channels)) {
+          GST_LOG_OBJECT (demux, "Ajusting AAC channels %d -> %d", channels,
+              adjusted_channels);
+        } else {
+          adjusted_channels = channels;
+        }
       }
       gst_buffer_unmap (demux->audio_codec_data, &map);
 
@@ -866,7 +877,7 @@ gst_flv_demux_audio_negotiate (GstFlvDemux * demux, guint32 codec_tag,
   }
 
   gst_caps_set_simple (caps, "rate", G_TYPE_INT, adjusted_rate,
-      "channels", G_TYPE_INT, channels, NULL);
+      "channels", G_TYPE_INT, adjusted_channels, NULL);
 
   if (demux->audio_codec_data) {
     gst_caps_set_simple (caps, "codec_data", GST_TYPE_BUFFER,