Correct mmsproxy and mmsport

There are no attributes called mmsc_proxy or mmsc_proxy_port.
Replace with mmsproxy and mmsport.

Test: Manual
Bug: TBD
Change-Id: I9e95667c9b7ea5f32fab692bbf10ad85777dbc73

etc/apns-full-conf.xml

@@ -38607,8 +38607,8 @@
       apn="enhancedphone"
       type="default,ia,mms,supl,fota"
       mmsc="http://mmsc.mobile.att.net"
-      mmsc_proxy="proxy.mobile.att.net"
-      mmsc_proxy_port="80"
+      mmsproxy="proxy.mobile.att.net"
+      mmsport="80"
       protocol="IPV4V6"
       roaming_protocol="IPV4V6"
       user_editable="false"
@@ -38647,8 +38647,8 @@
       apn="nrphone"
       type="default,ia,mms,supl,fota"
       mmsc="http://mmsc.mobile.att.net"
-      mmsc_proxy="proxy.mobile.att.net"
-      mmsc_proxy_port="80"
+      mmsproxy="proxy.mobile.att.net"
+      mmsport="80"
       protocol="IPV4V6"
       roaming_protocol="IPV4V6"
       user_editable="false"