Move flags to exported for g3 access

Bug: 297446980
Change-Id: If7c12a9d363f6d33451321f764afa36f83fd80b8
diff --git a/flags/calling.aconfig b/flags/calling.aconfig
index e67ebc6..f8de8b2 100644
--- a/flags/calling.aconfig
+++ b/flags/calling.aconfig
@@ -5,6 +5,7 @@
   namespace: "telephony"
   description: "APIs that are used to notify simultaneous calling changes to other applications."
   bug: "297446980"
+  is_exported: true
 }
 
 flag {