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/get/FinishedMission.java

@@ -12,5 +12,7 @@ public class FinishedMission extends  Mission {
         length = mission.length;// ¿or mission.done?
         timestamp = mission.timestamp;
         kind = mission.kind;
+        storage = mission.storage;
+
     }
 }