Adds request and response for handling network validation feature.

A new request network validation function has been added. Only QNS
can request this, and a network validation can be requested to each
DataNetwork through AccessNetworksManager. DataNetwork can request
validation to the DataServices such as Cellular or IWLAN through the
DataService API. If there is a DataService that does not support the
network validation feature, it may reply UNSUPPORTED as a default
result.
The Validation status variable has been added to DataCallResponse in
response to a network validation request. Network validation status
has also been added to PrecisedataConnectionState so that privileged
apps can use this variable to know.

Bug: 299346675
Test: atest FrameworksTelephonyTests
Change-Id: I77217967731ccadf9efdfe918d773ae8994f49d0
diff --git a/flags/data.aconfig b/flags/data.aconfig
index d65dc55..befb38d 100644
--- a/flags/data.aconfig
+++ b/flags/data.aconfig
@@ -41,3 +41,11 @@
   description: "Expose apn setting supporting field"
   bug: "307038091"
 }
+
+flag {
+  name: "network_validation"
+  namespace: "telephony"
+  description: "Request network validation for data networks and response status."
+  bug:"286171724"
+}
+