[mle] enable state update timer when enabling router role (#4094)
[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.
chshu/openthread,abtink/openthread,chshu/openthread,abtink/openthread,bukepo/openthread,librasungirl/openthread,jwhui/openthread,lanyuwen/openthread,bukepo/openthread,abtink/openthread,turon/openthread,librasungirl/openthread,bukepo/openthread,turon/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,jwhui/openthread,bukepo/openthread,srickardti/openthread,chshu/openthread,lanyuwen/openthread,librasungirl/openthread,srickardti/openthread,abtink/openthread,turon/openthread,chshu/openthread,jwhui/openthread,openthread/openthread,lanyuwen/openthread,srickardti/openthread,openthread/openthread,turon/openthread,openthread/openthread,srickardti/openthread,lanyuwen/openthread