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
diff --git a/src/com/android/settings/dream/DreamAdapter.java b/src/com/android/settings/dream/DreamAdapter.java
index 44f77e3..2237712 100644
--- a/src/com/android/settings/dream/DreamAdapter.java
+++ b/src/com/android/settings/dream/DreamAdapter.java
@@ -84,6 +84,9 @@
                 mPreviewView.setImageDrawable(previewImage);
                 mPreviewView.setClipToOutline(true);
                 mPreviewPlaceholderView.setVisibility(View.GONE);
+            } else {
+                mPreviewView.setImageDrawable(null);
+                mPreviewPlaceholderView.setVisibility(View.VISIBLE);
             }
 
             final Drawable icon = item.isActive()