Add aconfig flag to control call behavior for data-only service

Bug: 296097429
Test: atest FrameworksTelephonyTests
Test: Feature test with flag both on and off
Change-Id: Iff417cf0f1dfa7eaf13b90e8d693728fea7b36af
diff --git a/flags/subscription.aconfig b/flags/subscription.aconfig
index f7c2a62..d2c36d6 100644
--- a/flags/subscription.aconfig
+++ b/flags/subscription.aconfig
@@ -19,4 +19,11 @@
   namespace: "telephony"
   description: "Supports customized cellular service capabilities per subscription."
   bug: "296097429"
+}
+
+flag {
+  name: "data_only_service_allow_emergency_call_only"
+  namespace: "telephony"
+  description: "Support emergency call only for data only cellular service."
+  bug: "296097429"
 }
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