Add vonr_enabled to voice call metric

vonr_enabled help us to identify VoNR and EPSFB calls.

Bug: 288449751
Test: m, atest VoiceCallSessionStatsTest and  maunal test

Change-Id: I5375a64ba6e3f36d9e618dbef7198b5fcf40a395
diff --git a/flags/data.aconfig b/flags/data.aconfig
index 152660b..267048e 100644
--- a/flags/data.aconfig
+++ b/flags/data.aconfig
@@ -83,3 +83,10 @@
   description: "notify data activity changed for slot id"
   bug: "309896936"
 }
+
+flag {
+  name: "vonr_enabled_metric"
+  namespace: "telephony"
+  description: "Collect vonr status in voice call metric"
+  bug:"288449751"
+}