iface-modem-messaging: instead of mem1,mem2,mem3 use just a new default storage

There is no point in specifying a default 'mem1' memory storage, which is used
for reading/listing/deleting, as those are operations that need a specific
'mem1' set each time.

Also, there is no point in specifying separate default 'mem2' and 'mem3' memory
storages, specially because now we allow Sms.Store() to specify a storage.

So, we will now only have a 'default' memory storage, which is applicable for
both 'mem2' and 'mem3' (storing, sending from storage and deleting).

introspection/org.freedesktop.ModemManager1.Modem.Messaging.xml

@@ -107,5 +107,13 @@
     -->
     <property name="SupportedStorages" type="au" access="read" />
 
+    <!--
+        DefaultStorage:
+
+        A <link linkend="MMSmsStorage">MMSmsStorage</link> value,
+        specifying the storage to be used when receiving or storing SMS.
+    -->
+    <property name="DefaultStorage" type="u" access="read" />
+
   </interface>
 </node>