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/data/cache/ChapterCache.kt

@@ -168,11 +168,11 @@ class ChapterCache(private val context: Context) {
      * @param imageUrl url of image.
      * @return path of image.
      */
-    fun getImagePath(imageUrl: String): String? {
+    fun getImagePath(imageUrl: String): File? {
         try {
             // Get file from md5 key.
             val imageName = DiskUtils.hashKeyForDisk(imageUrl) + ".0"
-            return File(diskCache.directory, imageName).canonicalPath
+            return File(diskCache.directory, imageName)
         } catch (e: IOException) {
             return null
         }