fix memory leak in CompositeWithOpaqueBackground (#4150)

Found with Leak canary: The transformation ends up in Glide's memory
cache and leaks whole Activities through the view -> context reference.

This fixes the problem by removing the background detection logic (so
the view reference is no longer needed) and setting the background
directly instead. Looks exactly as before.

app/src/main/java/com/keylesspalace/tusky/adapter/StatusBaseViewHolder.java

@@ -335,9 +335,13 @@ public abstract class StatusBaseViewHolder extends RecyclerView.ViewHolder {
             avatarRadius = avatarRadius36dp;
         }
 
-        ImageLoadingHelper.loadAvatar(url, avatar, avatarRadius,
+        ImageLoadingHelper.loadAvatar(
+            url,
+            avatar,
+            avatarRadius,
             statusDisplayOptions.animateAvatars(),
-            Collections.singletonList(new CompositeWithOpaqueBackground(avatar)));
+            Collections.singletonList(new CompositeWithOpaqueBackground(MaterialColors.getColor(avatar, android.R.attr.colorBackground)))
+        );
     }
 
     protected void setMetaData(@NonNull StatusViewData.Concrete statusViewData, @NonNull StatusDisplayOptions statusDisplayOptions, @NonNull StatusActionListener listener) {