Add feature flag to clears cached IMS phone number when device lost IMS registration.

Bug: 307133015
Test: atest ImsPhoneTest, ImsServiceTest
Change-Id: I140ac7c5bbeea3e4f262b7415a883aef02982c0d
diff --git a/flags/ims.aconfig b/flags/ims.aconfig
index 4f06049..482802a 100644
--- a/flags/ims.aconfig
+++ b/flags/ims.aconfig
@@ -12,4 +12,11 @@
     namespace: "telephony"
     description: "This flag ignores the incoming call by throwing an exception if the call was already terminated before the framework registers the listener for the incoming call"
     bug:"289461637"
-}
\ No newline at end of file
+}
+
+flag {
+    name: "clear_cached_ims_phone_number_when_device_lost_ims_registration"
+    namespace: "telephony"
+    description: "This flag clears cached IMS phone number when device lost IMS registration"
+    bug:"288002989"
+}