Introduce aconfig flag to guard Reset mobile network feature

The flag is used in components below:
- API (an activity action) defined in TelephonyManager
- Implementation in Settings app
- Test cases

Bug: 271921464
Test: atest TelephonyManagerTest
Test: Feature functionality test
Change-Id: I60251fdc9a1040740c71519bcf4e56067093637d
diff --git a/flags/misc.aconfig b/flags/misc.aconfig
index ba9772b..dff6426 100644
--- a/flags/misc.aconfig
+++ b/flags/misc.aconfig
@@ -63,3 +63,10 @@
   description: "Expose carrier config KEY_ADDITIONAL_SETTINGS_CALLER_ID_VISIBILITY_BOOL and KEY_ADDITIONAL_SETTINGS_CALL_WAITING_VISIBILITY_BOOL."
   bug: "310264981"
 }
+
+flag {
+    name: "reset_mobile_network_settings"
+    namespace: "telephony"
+    description: "Allows applications to launch Reset Mobile Network Settings page in Settings app."
+    bug:"271921464"
+}