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;

app/src/main/java/org/schabi/newpipe/streams/WebMWriter.java

@@ -107,7 +107,7 @@ public class WebMWriter {
         parsed = true;
 
         for (SharpStream src : sourceTracks) {
-            src.dispose();
+            src.close();
         }
 
         sourceTracks = null;

app/src/main/java/org/schabi/newpipe/streams/io/SharpStream.java

@@ -1,11 +1,12 @@
 package org.schabi.newpipe.streams.io;
 
+import java.io.Closeable;
 import java.io.IOException;
 
 /**
  * based on c#
  */
-public abstract class SharpStream {
+public abstract class SharpStream  implements Closeable {
 
     public abstract int read() throws IOException;
 
@@ -19,9 +20,10 @@ public abstract class SharpStream {
 
     public abstract void rewind() throws IOException;
 
-    public abstract void dispose();
+    public abstract boolean isClosed();
 
-    public abstract boolean isDisposed();
+    @Override
+    public abstract void close();
 
     public abstract boolean canRewind();
 
@@ -54,4 +56,8 @@ public abstract class SharpStream {
     public void seek(long offset) throws IOException {
         throw new IOException("Not implemented");
     }
+
+    public long length() throws IOException {
+        throw new UnsupportedOperationException("Unsupported operation");
+    }
 }