Add GalleryProvider.cancelRequest(int), fix SpiderQueen.request(int, boolean)

app/src/main/java/com/hippo/ehviewer/gallery/GalleryProvider.java

@@ -78,6 +78,8 @@ public abstract class GalleryProvider {
         request(index);
     }
 
+    public abstract void cancelRequest(int index);
+
     public abstract String getError();
 
     public void setGalleryProviderListener(GalleryProviderListener listener) {