us: mark primary provider for MCC/MNC 310/410

BUG=chromium:311893
TEST=xmllint --valid --noout serviceproviders.xml
TEST=Run unit tests
TEST=Verify that the primary provider for MCC/MNC 310/410 is selected by shill by default.

Change-Id: I05d3a68544c7906ec9ee0eaeb334aead7c7a84c7
Reviewed-on: https://chromium-review.googlesource.com/174833
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
diff --git a/serviceproviders.xml b/serviceproviders.xml
index 30ab630..2ccf279 100644
--- a/serviceproviders.xml
+++ b/serviceproviders.xml
@@ -10436,7 +10436,7 @@
 
 <!-- United States -->
 <country code="us">
-	<provider>
+	<provider primary="true">
 		<name>AT&amp;T</name>
 		<gsm>
 			<network-id mcc="310" mnc="038"/>