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-contacts.c

@@ -14,9 +14,8 @@
 static void
 test_contacts_null_contact (void)
 {
-  g_autoptr (CallsSettings) settings = calls_settings_new ();
   g_autoptr (CallsContactsProvider) contacts_provider =
-    calls_contacts_provider_new (settings);
+    calls_contacts_provider_new ();
   CallsBestMatch *best_match;
 
   best_match = calls_contacts_provider_lookup_id (contacts_provider, NULL);