Refuse to start app on Android 4 beyond expiry date
When the expiry date for Android 4 has been reached the app should refuse to start. This can use a similar mechanism to the existing ExpiredActivity for debug builds.
We might want to provide a button that deletes the user's account, if that's easy to achieve, or just let the user know that their account will be deleted when they uninstall the app.
Subtask of #2221.