Implement Storage Access Framework

* re-work finished mission database
* re-work DownloadMission and bump it Serializable version
* keep the classic Java IO API
* SAF Tree API support on Android Lollipop or higher
* add wrapper for SAF stream opening
* implement Closeable in SharpStream to replace the dispose() method

* do required changes for this API:
** remove any file creation logic from DownloadInitializer
** make PostProcessing Serializable and reduce the number of iterations
** update all strings.xml files
** storage helpers: StoredDirectoryHelper & StoredFileHelper
** best effort to handle any kind of SAF errors/exceptions

app/src/main/java/org/schabi/newpipe/player/playback/MediaSourceManager.java

@@ -158,7 +158,7 @@ public class MediaSourceManager {
      * Dispose the manager and releases all message buses and loaders.
      * */
     public void dispose() {
-        if (DEBUG) Log.d(TAG, "dispose() called.");
+        if (DEBUG) Log.d(TAG, "close() called.");
 
         debouncedSignal.onComplete();
         debouncedLoader.dispose();