Update Straight Talk ATT MMS proxy

Using proxy.mvno.tracfone.com should also future proof it if they change the
IP again. proxy.mvno.tracfone.com resolves to (66.209.11.32)

Change-Id: I8cb5b3ce473bef1369ec4ee313759258fc1172d3
diff --git a/prebuilt/common/etc/apns-conf.xml b/prebuilt/common/etc/apns-conf.xml
index 09a2319..87fb1e0 100644
--- a/prebuilt/common/etc/apns-conf.xml
+++ b/prebuilt/common/etc/apns-conf.xml
@@ -579,7 +579,7 @@
   <apn carrier="ATT LTE" mcc="310" mnc="410" apn="pta" mmsc="http://mmsc.mobile.att.net" mmsproxy="proxy.mobile.att.net" mmsport="80" type="default,supl,mms" />
   <apn carrier="ATT Broadband" mcc="310" mnc="410" apn="broadband" user="" server="" password="" mmsc="" type="default" />
   <apn carrier="ATT Activation" mcc="310" mnc="410" apn="lwaactivate" user="" server="" password="" mmsc="" protocol="IP" />
-  <apn carrier="StraightTalk ATT" mcc="310" mnc="410" apn="att.mvno" mmsc="http://mmsc.cingular.com" mmsproxy="66.209.11.33" mmsport="80" type="default,supl,mms" />
+  <apn carrier="StraightTalk ATT" mcc="310" mnc="410" apn="att.mvno" mmsc="http://mmsc.cingular.com" mmsproxy="proxy.mvno.tracfone.com" mmsport="80" type="default,supl,mms" />
   <apn carrier="CBW Internet" apn="wap.gocbw.com" proxy="" port="" user="cbw" password="" server="216.68.79.199" mmsc="http://mms.gocbw.com:8088/mms" mmsproxy="216.68.79.202" mmsport="80" mcc="310" mnc="420" type="default,supl,mms" />
   <apn carrier="Viero" mcc="310" mnc="450" apn="internet.vedge.com" proxy="10.168.3.23" port="9401" type="default,supl" />
   <apn carrier="Viero MMS" mcc="310" mnc="450" apn="mms" mmsc="http://mms.viero.com" mmsproxy="10.168.3.23" mmsport="9401" type="mms" />