Move local/remote playlist merge() to PlaylistLocalItem class

In order not to have a utils class just for one function

app/src/main/java/org/schabi/newpipe/database/playlist/PlaylistLocalItem.java

@@ -1,7 +1,33 @@
 package org.schabi.newpipe.database.playlist;
 
 import org.schabi.newpipe.database.LocalItem;
+import org.schabi.newpipe.database.playlist.model.PlaylistRemoteEntity;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
 public interface PlaylistLocalItem extends LocalItem {
     String getOrderingName();
+
+    static List<PlaylistLocalItem> merge(
+            final List<PlaylistMetadataEntry> localPlaylists,
+            final List<PlaylistRemoteEntity> remotePlaylists) {
+        final List<PlaylistLocalItem> items = new ArrayList<>(
+                localPlaylists.size() + remotePlaylists.size());
+        items.addAll(localPlaylists);
+        items.addAll(remotePlaylists);
+
+        Collections.sort(items, (left, right) -> {
+            final String on1 = left.getOrderingName();
+            final String on2 = right.getOrderingName();
+            if (on1 == null) {
+                return on2 == null ? 0 : 1;
+            } else {
+                return on2 == null ? -1 : on1.compareToIgnoreCase(on2);
+            }
+        });
+
+        return items;
+    }
 }