Separate imageListToDbUrl from choosePreferredImage

imageListToDbUrl should be used if the URL is going to be saved to the database, to avoid saving nothing in case at the moment of saving the user preference is to not show images.

app/src/main/java/org/schabi/newpipe/fragments/list/channel/ChannelFragment.java

@@ -355,7 +355,7 @@ public class ChannelFragment extends BaseStateFragment<ChannelInfo>
                 channel.setServiceId(info.getServiceId());
                 channel.setUrl(info.getUrl());
                 channel.setData(info.getName(),
-                        ImageStrategy.choosePreferredImage(info.getAvatars()),
+                        ImageStrategy.imageListToDbUrl(info.getAvatars()),
                         info.getDescription(),
                         info.getSubscriberCount());
                 channelSubscription = null;