[network-diagnostics] use the child timeout directly (#4136)

This commit changes the `NetworkDiagnostic::FillRequestedTlvs()` to
directly use the child timeout value in appended `TimeoutTlv`. This
addresses an issue where a user-specified long data poll interval
could cause the timeout TLV in diags response to be set higher than
actual child's timeout value.

src/core/thread/network_diagnostic.cpp

@@ -336,7 +336,7 @@ otError NetworkDiagnostic::FillRequestedTlvs(Message &             aRequest,
             {
                 TimeoutTlv tlv;
                 tlv.Init();
-                tlv.SetTimeout(TimerMilli::MsecToSec(Get<DataPollSender>().GetKeepAlivePollPeriod()));
+                tlv.SetTimeout(Get<Mle::MleRouter>().GetTimeout());
                 SuccessOrExit(error = aResponse.AppendTlv(tlv));
             }