Improve DownloadManager and -Service

* Fix permission at some places
 * Fix access problem for downloaded files with external player
 * Store finished Downloads
 * Remove binding to DownloadService just to download a file
 * Javadoc
 * Code improvements

app/src/main/java/us/shandian/giga/get/DownloadDataSource.java

@@ -0,0 +1,36 @@
+package us.shandian.giga.get;
+
+import java.util.List;
+
+/**
+ * Provides access to the storage of {@link DownloadMission}s
+ */
+public interface DownloadDataSource {
+
+    /**
+     * Load all missions
+     * @return a list of download missions
+     */
+    List<DownloadMission> loadMissions();
+
+    /**
+     * Add a downlaod mission to the storage
+     * @param downloadMission the download mission to add
+     * @return the identifier of the mission
+     */
+    void addMission(DownloadMission downloadMission);
+
+    /**
+     * Update a download mission which exists in the storage
+     * @param downloadMission the download mission to update
+     * @throws IllegalArgumentException if the mission was not added to storage
+     */
+    void updateMission(DownloadMission downloadMission);
+
+
+    /**
+     * Delete a download mission
+     * @param downloadMission the mission to delete
+     */
+    void deleteMission(DownloadMission downloadMission);
+}