Initial skeleton code for a SyncAdapter.

Based on
http://developer.android.com/training/sync-adapters/index.html
LoginActvity, ItemsContentProvider: Store the login details in the AccountManager.

Instead of in SharedPreferences.

Conflicts:
Add accounts permissions.

Otherwise we are not allowed to get our own account details and our own
credentials from that account.
Authenticator: Implement addAccount() so the user can add an account via Settings.
Remove unnecessary imports.
Remove unused preference keys.
Remove decrypt/encrypt tests of API that we removed.
ItemsContentProvider: Move all network IO to SyncAdapter.

This is not quite working properly yet, and we don't trigger it at the right times
yet.

At the least, it's nice to separate the database and network code, even if we use the nasty
ContentProvider API to do it.
Remove unused code.
ItemsContentProvider: Remove unused parameters.
SyncAdapter: Make a constant private.
ClassifyFragment: Warn (and don't crash) if we cannot get a Next item.

This can happen if the network is connected but is not really working.
ZooniverseClient: Raise the maximum count of items to request.

Because our SyncAdapter seems to manage this much better, with no noticeable
effect on the UI responsiveness.
UiUtils: Move error messages to strings.xml so we can translate them.
SubjectFragment: showImage(): Fix a typo in the check
ClassifyActivity, LoginActivity: Always have an anonymous Account.

Because otherwise our SyncAdapter will never run.
Unfortunately we have to delete the account when loggin in with a real account name
(or when logging in with a different account name) so we lose previous data
(though it remains on the web site under your profile).
SyncAdapter: Respond to preference changes.

Like the ItemsContentProvider used to.
ItemsContentProvider: Ask our SyncAdaptor to do work when appropriate.

Call our requestSync() wherever we previously called queueRegularTasks(),
when it's likely that an upload, download, or delete could be necessary.
SyncAdapter: Move some code into a SubjectAdder class so ItemsContentProvider can use it too.

ItemsContentProvider needs to get and add a subject (and its images) immediately, and
synchronously, in the unusual (maybe initial) case that the SyncAdapter has not added any yet.
fix