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/AndroidManifest.xml

@@ -161,6 +161,16 @@
         <activity
             android:name=".ReCaptchaActivity"
             android:label="@string/reCaptchaActivity" />
+
+        <provider
+            android:name="android.support.v4.content.FileProvider"
+            android:authorities="${applicationId}.provider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/provider_paths"/>
+        </provider>
     </application>
 
 </manifest>