Download manager rewrite (#535)

* Saving to SD working

* Rename imagePath to uri

* Handle android < 21

* Minor changes

* Separate downloader from the manager. Optimize folder lookups

* Persist downloads across restarts

* Fix for #511

* Updated ReactiveNetwork. Add some documentation

* More documentation and minor fixes

* Handle persistent notifications. Other minor changes

* Improve downloader and add documentation

* Rename pageNumber to index in Page class

* Remove unused methods

* Use chop method

* Make sure dest dir is created

* Reset downloads dir preference

* Use invalidate options menu in download fragment and fix wrong condition

* Fix empty download queue after application restart

* Use addAll method in download queue to avoid too many notifications

* Inform download manager changes

app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryPresenter.kt

@@ -185,15 +185,10 @@ class LibraryPresenter : BasePresenter<LibraryFragment>() {
             }
 
             if (prefFilterDownloaded) {
-                val mangaDir = downloadManager.getAbsoluteMangaDirectory(source, manga)
+                val mangaDir = downloadManager.findMangaDir(source, manga)
 
-                if (mangaDir.exists()) {
-                    for (file in mangaDir.listFiles()) {
-                        if (file.isDirectory && file.listFiles().isNotEmpty()) {
-                            hasDownloaded = true
-                            break
-                        }
-                    }
+                if (mangaDir != null) {
+                    hasDownloaded = mangaDir.listFiles()?.any { it.isDirectory } ?: false
                 }
             }