[mle] fix bug in restarting router state update timer (#2472)

This commit moves restarting the state update timer to the top of
`MleRouter::HandleStateUpdateTimer()`.  This ensures that the timer
continues to fire periodically.  Prior to this change, the state update
timer is not properly restarted when `mRouterSelectionJitterTimeout`
expires.

src/core/thread/mle_router.cpp

@@ -442,7 +442,6 @@ otError MleRouter::SetStateRouter(uint16_t aRloc16)
     mRouters[mRouterId].SetNextHop(mRouterId);
     mPreviousPartitionId = mLeaderData.GetPartitionId();
     netif.GetNetworkDataLeader().Stop();
-    mStateUpdateTimer.Start(kStateUpdatePeriod);
     netif.GetIp6().SetForwardingEnabled(true);
     netif.GetIp6().GetMpl().SetTimerExpirations(kMplRouterDataMessageTimerExpirations);
     netif.GetMac().SetBeaconEnabled(true);
@@ -1792,6 +1791,8 @@ void MleRouter::HandleStateUpdateTimer(void)
 {
     bool routerStateUpdate = false;
 
+    mStateUpdateTimer.Start(kStateUpdatePeriod);
+
     if (mChallengeTimeout > 0)
     {
         mChallengeTimeout--;
@@ -1986,8 +1987,6 @@ void MleRouter::HandleStateUpdateTimer(void)
 
     SynchronizeChildNetworkData();
 
-    mStateUpdateTimer.Start(kStateUpdatePeriod);
-
 exit:
     return;
 }