ClassifyActivity: Don't call LoginUtils.addAnonymousAccount() from main UI thread.

Because it uses the AccountManager, which we should not use from a UI thread.
SubjectFragment: Avoid calling BitmapFactory.decodeStream() in main thread.

Move the code into an AsyncTask.
SubjectFragment: ShowImageTask(): Let this work for any ItemFragment.

And rename it to ShowImageFromContentProviderTask.