Change default of saved-im-presence to Offline

Unset is not a valid presence type so the Shell shouldn't try setting it when
starting for the first time.

https://bugzilla.gnome.org/show_bug.cgi?id=661272

data/org.gnome.shell.gschema.xml.in

@@ -51,7 +51,7 @@
       <_summary>History for the looking glass dialog</_summary>
     </key>
     <key name="saved-im-presence" type="i">
-      <default>0</default>
+      <default>1</default>
       <_summary></_summary>
     </key>
     <key name="saved-session-presence" type="i">