PlayerView: Provide option to show default artwork when no prepared player is attached

hi
I wanna show a preview bitmap of a video in exoplayer.i have the bitmap and I use
```
playerview.setUseArtwork(true);
playerview.setDefaultArtwork(bitmap);
```
but it doesn't work.
***i havent write any code to connect playerview to exoplayer.
I used an image view to show the preview and then hide it but it wasn't good.