more SAF implementation

* full support for Directory API (Android Lollipop or later)
* best effort to handle any kind errors (missing file, revoked permissions, etc) and recover the download
* implemented directory choosing
* fix download database version upgrading
* misc. cleanup
* do not release permission on the old save path (if the user change the download directory) under SAF api

app/src/main/java/us/shandian/giga/io/FileStreamSAF.java

@@ -137,4 +137,9 @@ public class FileStreamSAF extends SharpStream {
     public void seek(long offset) throws IOException {
         channel.position(offset);
     }
+
+    @Override
+    public long length() throws IOException {
+        return channel.size();
+    }
 }