Emergency fix for quit when stack is not empty

Fixes:https://github.com/asuka-mio/Ehviewer/commit/10caf7bbd8472dabe616733ad92aafaf89b2a9ee

app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryListScene.java

@@ -844,7 +844,8 @@ public final class GalleryListScene extends BaseScene
 
         switch (mState) {
             case STATE_NORMAL:
-                requireActivity().moveTaskToBack(false);
+                if (getStackIndex() == 0)
+                    requireActivity().moveTaskToBack(false);
                 break;
             case STATE_SIMPLE_SEARCH:
             case STATE_SEARCH: