Fix download progress indicator (#8188)

(cherry picked from commit b1e104319f90678bc7ed501797d9d7d5b98902b5)

app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt

@@ -322,13 +322,14 @@ class Downloader(
         val pageListObservable = if (download.pages == null) {
             // Pull page list from network and add them to download object
             download.source.fetchPageList(download.chapter)
-                .doOnNext { pages ->
+                .map { pages ->
                     if (pages.isEmpty()) {
                         throw Exception(context.getString(R.string.page_list_empty_error))
                     }
                     // Don't trust index from source
                     val reIndexedPages = pages.mapIndexed { index, page -> Page(index, page.url, page.imageUrl, page.uri) }
                     download.pages = reIndexedPages
+                    reIndexedPages
                 }
         } else {
             // Or if the page list already exists, start from the file