Clean up download ahead logic

- Remove redundant chapter sorting logic when fetching next chapter(s)
- Remove redundant download queue checks (it'll handle already queued or downloaded items)
- Trigger download ahead when read >= 25% of chapter rather than 20%
- Rely on download cache when checking if next chapter is downloaded to avoid jank (fixes #8328)

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

@@ -244,6 +244,10 @@ class Downloader(
      * @param autoStart whether to start the downloader after enqueing the chapters.
      */
     fun queueChapters(manga: Manga, chapters: List<Chapter>, autoStart: Boolean) = launchIO {
+        if (chapters.isEmpty()) {
+            return@launchIO
+        }
+
         val source = sourceManager.get(manga.source) as? HttpSource ?: return@launchIO
         val wasEmpty = queue.isEmpty()
         // Called in background thread, the operation can be slow with SAF.

app/src/main/java/eu/kanade/tachiyomi/data/library/LibraryUpdateService.kt

@@ -415,8 +415,7 @@ class LibraryUpdateService(
     private fun downloadChapters(manga: Manga, chapters: List<Chapter>) {
         // We don't want to start downloading while the library is updating, because websites
         // may don't like it and they could ban the user.
-        val dbChapters = chapters.map { it.toDbChapter() }
-        downloadManager.downloadChapters(manga, dbChapters, false)
+        downloadManager.downloadChapters(manga, chapters.map { it.toDbChapter() }, false)
     }
 
     /**

app/src/main/java/eu/kanade/tachiyomi/data/notification/NotificationReceiver.kt

@@ -271,11 +271,9 @@ class NotificationReceiver : BroadcastReceiver() {
      */
     private fun downloadChapters(chapterUrls: Array<String>, mangaId: Long) {
         launchIO {
-            val manga = getManga.await(mangaId)
+            val manga = getManga.await(mangaId) ?: return@launchIO
             val chapters = chapterUrls.mapNotNull { getChapter.await(it, mangaId)?.toDbChapter() }
-            if (manga != null && chapters.isNotEmpty()) {
-                downloadManager.downloadChapters(manga, chapters)
-            }
+            downloadManager.downloadChapters(manga, chapters)
         }
     }