Fix some crashes introduced in #461

Prevent crashes in `onPostExecute()` methods of DeckTasks by not calling `showDialogFragment()`
Force `startLoadingCollection()` to re-sync with `AnkiDroidApp.getCol()` by calling `resetLoader()` instead of `initLoader()`

http://ankidroid-triage.appspot.com/view_crash?crash_id=5013690914963456
http://ankidroid-triage.appspot.com/view_crash?crash_id=5282921745219584