ManageSpaceActivity: kill the app after finishing cleanup

This will prevent the file list still being on the Recents view and other stuff like that.

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

src/main/java/com/owncloud/android/ui/activity/ManageSpaceActivity.java

@@ -137,7 +137,8 @@ public class ManageSpaceActivity extends AppCompatActivity implements Injectable
                         Snackbar.LENGTH_LONG
                 ).show();
             } else {
-                finish();
+                finishAndRemoveTask();
+                System.exit(0);
             }
         }