OpusDecoder conflicts with opus library.

[mateofio]

Our OpusDecoder uses the same name as struct OpusDecoder on the system opus.h which can cause a conflict.

So far it seems benign, but we should fix it. The proper solution is to namespace everything.

Discovered by a warning on android build:

In file included from jni/src/../../../../../../..//src/audio_decoder.cpp:30:
jni/src/../../../../../../..//src/decoder_opus.h:29:1: warning: 'OpusDecoder' defined as a class here but previously declared as a struct [-Wmismatched-tags]
class OpusDecoder : public AudioDecoder {
^
/var/lib/jenkins/workspace/player-android-pr/../toolchain-android/android/armeabi-v7a-toolchain/include/opus/opus.h:399:9: note: did you mean class here?
typedef struct OpusDecoder OpusDecoder;
        ^~~~~~
        class
1 warning generated.