[mle] skip saving NetworkInfo before first device attach (#3870)
[mle] skip saving NetworkInfo before first device attach (#3870)

This commit changes the `Mle::Store()` behavior such that if a device
is not attached and there is no previously saved `NetworkInfo`
(indicating that the device has never been attached before) we skip
saving/updating the `NetworkInfo`. Note that on a device reset the
MLE/MAC frame counters (which are part of `NetworkInfo`) would reset
but device would also start with a new randomly generated extended
address (so the security nonces would not be repeated).
abtink/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,bukepo/openthread,bukepo/openthread,srickardti/openthread,openthread/openthread,turon/openthread,librasungirl/openthread,turon/openthread,jwhui/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,abtink/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread,librasungirl/openthread,openthread/openthread,chshu/openthread,srickardti/openthread,jwhui/openthread,bukepo/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,bukepo/openthread,jwhui/openthread,turon/openthread,chshu/openthread,turon/openthread,srickardti/openthread,lanyuwen/openthread