[mle] use local ref to KeyManager/Mac/MleRouter for reused objects in methods (#3551)
[mle] use local ref to KeyManager/Mac/MleRouter for reused objects in methods (#3551)


librasungirl/openthread,openthread/openthread,lanyuwen/openthread,lanyuwen/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,bukepo/openthread,bukepo/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,lanyuwen/openthread,lanyuwen/openthread,jwhui/openthread,jwhui/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,turon/openthread,chshu/openthread,jwhui/openthread,turon/openthread,jwhui/openthread,srickardti/openthread,turon/openthread,bukepo/openthread,chshu/openthread,srickardti/openthread,turon/openthread,openthread/openthread,openthread/openthread,chshu/openthread,librasungirl/openthread,bukepo/openthread,openthread/openthread,librasungirl/openthread,abtink/openthread