decoder_finder: force libopus for opus decoding

ffmpeg implements two opus decoders. libopusdec is a wrapper around the
libopus reference opus library. opusdec is a native implementation, but
lacks features such as FEC support in some distribution. Depending on
the distribution configuration, ffmpeg can select either decoder for
opus.

This patch forces the use of libopus if it exists. If it does not, the
function will fall back to opusdec.

Issue: #4857
Change-Id: Ia1838a593c9199fc1cbda5f4596c318a8cdd886b

src/media/decoder_finder.h

@@ -46,6 +46,8 @@ findDecoder(const enum AVCodecID codec_id)
     case AV_CODEC_ID_VP9:
         codec_name = "vp9_mediacodec"; break;
 #endif
+    case AV_CODEC_ID_OPUS:
+        codec_name = "libopus"; break;
     default:
         codec_name = nullptr;
     }