Support retrieving notices from URL (Google Play version)

[TheLastProject]

Google Play is an incredibly developer-hostile platform. We cannot trust Catima to be available there long-term. We should be able to somehow communicate with Catima users on Google Play. For example: check a pre-defined URL on start in a thread at most once per X time (allow it to fail) and if a message is there, show it to the user.

A good option would be to return JSON with an ID in it and store the last-shown ID to only show it to users once.