Support handover rule based on incall

Some carriers do not allow in-call handover such as NR and WiFi
due to network limitation. Adding the incall rule to support it.

Flag: com.android.internal.telephony.flags.incall_handover_policy
Bug: 376765521
Test: atest DataNetworkControllerTest
Change-Id: Ia1e6b367fa212d464d692d7e6b9d8ab429a3c066
diff --git a/flags/data.aconfig b/flags/data.aconfig
index 17d1adb..f3409f8 100644
--- a/flags/data.aconfig
+++ b/flags/data.aconfig
@@ -129,3 +129,14 @@
   bug: "366194627"
   is_exported: true
 }
+
+# OWNER=gwenlin TARGET=25Q2
+flag {
+  name: "incall_handover_policy"
+  namespace: "telephony"
+  description: "Support IWLAN handover policy based on incall."
+  bug:"376765521"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
+}