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/streams/Mp4FromDashWriter.java

@@ -120,7 +120,7 @@ public class Mp4FromDashWriter {
         parsed = true;
 
         for (SharpStream src : sourceTracks) {
-            src.dispose();
+            src.close();
         }
 
         tracks = null;