getPlaybackType() {
    return Playback.NO_OP
  }
gets the playback type (`'vod', 'live', 'aod'`)
@method getPlaybackType
@return {String} you should write the playback type otherwise it'll assume `'no_op'`
@example
```javascript
html5VideoPlayback.getPlaybackType() //vod
html5AudioPlayback.getPlaybackType() //aod
html5VideoPlayback.getPlaybackType() //live
flashHlsPlayback.getPlaybackType() //live
```