Bring back exported FR flag data_only_cellular_service

The exported FR flag data_only_cellular_service for 24Q3 was removed
from the aconfig file by accident when cleaning up the flag. It will
cause some CTS failures.

According the go/android-flags-guidelines#flag-end-of-life, an exported
FR flag should never be removed from the aconfig file.

Bug: 377864488
Test: m -j
Flag: EXEMPT flag change only
Change-Id: I3828a7cdeb3ff35b9f9bc55ef44b9d6b481cf51a
diff --git a/flags/subscription.aconfig b/flags/subscription.aconfig
index 543aa20..76485be 100644
--- a/flags/subscription.aconfig
+++ b/flags/subscription.aconfig
@@ -19,6 +19,15 @@
   bug: "296076674"
 }
 
+# OWNER=rambowang TARGET=24Q3
+flag {
+  name: "data_only_cellular_service"
+  is_exported: true
+  namespace: "telephony"
+  description: "Supports customized cellular service capabilities per subscription."
+  bug: "296097429"
+}
+
 # OWNER=hhshin TARGET=24Q3
 flag {
   name: "support_psim_to_esim_conversion"