Remove an outdated TODO comment.
ItemsContentProvider: UploadAsyncTask.doInBackground(): Extract a method to make it clearer.
ItemsContentProvider.requestMoreItemsSync(): Do not request too many.

In order to avoid too much network and disk IO all at the same time
as we download the images. This can make the UI non responsive.
It doesn't seem to help to do it in a Thread with Thread.MIN_PRIORITY
either - too much IO will just slow the UI down.
ItemsContentProvider.requestMoreItemsSync(): Do not request too many at once.

This slightly avoids the problem of buttons not responding during image
downloads or classification uploads, but not completely.
ItemsContentProvider.query(): Don't leak a Cursor.
ItemsContentProvider: Respond to preferences changes.

For instance, to start removing old items or downloading new items
if the preference has changed.
ItemsContentProvider: Do the initial queueRegularTasks in onCreate.

Instead of in the constructor, which could lead to us needing the Context
before getContext() will return non-null.