fix
LoginActivity: Remove incorrect comments.
SubjectFragment, ListCursorAdapter: Abandon a subject if we cannot load one of its images.
ClassifyActivity: listen to preferences changes here instead of in SyncAdapater.

Because it doesn't work in SyncAdapter when the SyncAdapter is in its own
process (when AndroidManifest.xml's <service> for it has
android:process=":sync" ). This is semi-documented: The SharedPreferences
docuementation says
"Note: currently this class does not support use across multiple processes. This will be added later."
However, do _get_ the correct preferences values in the SyncAdapter even
if we cannot listen for changes to them.
ClassifyActivity: Improve some comments.
ClassifyActivity: Don't bother making the SyncAdapter respond to all ContentProvider changes.

Because our ItemsContentProvider requests a sync when it is necessary.
Remove unnecessary imports.
SubjectAdder: Use <> to instantiate generics.
Make some fields final.
SubjectFragment: Fix a typo in a null/empty check.