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);

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);

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;
 }

tests/test-ringer.c

@@ -138,7 +138,7 @@ __wrap_lfb_event_end_feedback_finish (LfbEvent     *self,
 
 
 CallsContactsProvider *
-__wrap_calls_contacts_provider_new (CallsSettings *settings)
+__wrap_calls_contacts_provider_new (void)
 {
   return NULL;
 }

tests/test-ui-call.c

@@ -14,7 +14,7 @@
 #include <gtk/gtk.h>
 
 CallsContactsProvider *
-__wrap_calls_contacts_provider_new (CallsSettings *settings)
+__wrap_calls_contacts_provider_new (void)
 {
   return NULL;
 }