Fix bug where dream preview image s not reset.

When the view is recycled, there may be an old preview image in the view
which should be cleared.

Bug: 222705951
Test: locally on device
Change-Id: I4bb1c176a9a6d04fd41780fa2da09ca685b87d26

src/com/android/settings/dream/DreamAdapter.java

@@ -84,6 +84,9 @@ public class DreamAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder>
                 mPreviewView.setImageDrawable(previewImage);
                 mPreviewView.setClipToOutline(true);
                 mPreviewPlaceholderView.setVisibility(View.GONE);
+            } else {
+                mPreviewView.setImageDrawable(null);
+                mPreviewPlaceholderView.setVisibility(View.VISIBLE);
             }
 
             final Drawable icon = item.isActive()