settings: Convert to the singleton pattern

We only have a single source of settings, so we should reflect that by
using a singleton. This also reduces our LoC.

This doesn't impair our ability to run tests because there we run with
GSETTINGS_BACKEND=memory

tests/mock-contacts-provider.h

@@ -11,4 +11,4 @@
 
 #include "calls-contacts-provider.h"
 
-CallsContactsProvider *__wrap_calls_contacts_provider_new (CallsSettings *settings);
+CallsContactsProvider *__wrap_calls_contacts_provider_new (void);