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/test-manager.c

@@ -12,7 +12,7 @@
 #include <libpeas/peas.h>
 
 CallsContactsProvider *
-__wrap_calls_contacts_provider_new (CallsSettings *settings)
+__wrap_calls_contacts_provider_new (void)
 {
   return NULL;
 }