title: File not Found Exception occurs when accessing flutter assets
body: <p>i am trying to write a plugin to play audio files that are stored in assets folder of flutter package and have done like this </p>
  
  <pre><code>if(call.method.equals("playMusic"))
  {
      Log.d(TAG, "onMethodCall: play music function called");
  
      String fileLocation = call.argument("file");
      Log.d(TAG, "onMethodCall: file requested is "+fileLocation);
      AssetManager assetManager = registrar.context().getAssets();
      String key = registrar.lookupKeyForAsset(fileLocation);
      Log.d(TAG, "onMethodCall: key is "+key);
      AssetFileDescriptor fd;
      MediaPlayer mediaPlayer = new MediaPlayer();
      try {
          Log.d(TAG, "onMethodCall: found assets " + Arrays.toString(assetManager.list("assets/")));
          fd= assetManager.openFd(key);
          mediaPlayer.setDataSource(fd.getFileDescriptor(),fd.getStartOffset(),fd.getLength());
          fd.close();
          mediaPlayer.prepare();
          mediaPlayer.start();
          result.success("played successfully");
    }
    catch (Exception e){
      Log.d(TAG, "onMethodCall: exception occured "+e.toString());
      result.success("playing failed");
    }
  }
  </code></pre>
  
  <p>fileLocation is correctly passed as </p>
  
  <blockquote>
    <p>assets/river.m4a</p>
  </blockquote>
  
  <p>i have checked and found out that the key which is looked up by registrar is </p>
  
  <blockquote>
    <p>flutter_assets/assets/river.m4a</p>
  </blockquote>
  
  <p>and the file is present in package at </p>
  
  <blockquote>
    <p>assets/flutter_assets/assets/river.m4a</p>
  </blockquote>
  
  <p>but still when i run the application it throws </p>
  
  <blockquote>
    <p>D/TunePlugin: onMethodCall: exception occured java.io.FileNotFoundException: flutter_assets/assets/river.m4a</p>
  </blockquote>