由于ijkplayer不能识别android.resource类型的资源在播放raw中的文件的时候用IjkMediaPlayer不能正常播放，实现IMediaDataSource为IjkMediaPlayer提供资源。

class RawDataSourceProvider implements IMediaDataSource{    AssetFileDescriptor mDescriptor;    byte[]  mMediaBytes;    long mPosition;    public RawDataSourceProvider(AssetFileDescriptor descriptor) {        this.mDescriptor = descriptor;    }    @Override    public int readAt(long position, byte[] buffer, int offset, int size) throws IOException {        if(position + 1 >= mMediaBytes.length){            return -1;        }        int length;        if(position + size < mMediaBytes.length){            length = size;        }else{            length = (int) (mMediaBytes.length - position);            if(length > buffer.length)                length = buffer.length ;            length--;        }        System.arraycopy(mMediaBytes, (int) position, buffer, offset, length);        mPosition = position;        return length;    }    @Override    public long getSize() throws IOException {        long length  = mDescriptor.getLength();        if(mMediaBytes == null){            Source source = Okio.source(mDescriptor.createInputStream());            mMediaBytes = Okio.buffer(source).readByteArray();        }        return length;    }    @Override    public void close() throws IOException {        if(mDescriptor != null)            mDescriptor.close();        mDescriptor = null;        mMediaBytes = null;    }}