[mle] enable state update timer when enabling router role (#4094)

The state update timer should be started when enabling router role and
attached to a network.

src/core/thread/mle_router.cpp

@@ -125,6 +125,11 @@ void MleRouter::SetRouterRoleEnabled(bool aEnabled)
 
         break;
     }
+
+    if (IsRouterRoleEnabled() && IsAttached() && !mStateUpdateTimer.IsRunning())
+    {
+        mStateUpdateTimer.Start(kStateUpdatePeriod);
+    }
 }
 
 otError MleRouter::BecomeRouter(ThreadStatusTlv::Status aStatus)