[mle] updates to advertise timer management (#2478)
[mle] updates to advertise timer management (#2478)

This commit fixes the following issues:

1. The advertise timer is no longer stopped when sending an Address Solicit
   message.  This ensures that a REED continues to send MLE Advertisements.

2. The advertise timer is always started after becoming a child, even when
   the router upgrade threshold is not yet met.  This ensures that a REED
   will start sending MLE Advertisements.
librasungirl/openthread,lanyuwen/openthread,pvanhorn/openthread,pvanhorn/openthread,pvanhorn/openthread,turon/openthread,gandreello/openthread,turon/openthread,pvanhorn/openthread,LeZhang2016/openthread,bukepo/openthread,mszczodrak/openthread,srickardti/openthread,gandreello/openthread,georgecpr/openthread,turon/openthread,mszczodrak/openthread,erja-gp/openthread,srickardti/openthread,gandreello/openthread,lanyuwen/openthread,gandreello/openthread,bukepo/openthread,gandreello/openthread,openthread/openthread,jwhui/openthread,lanyuwen/openthread,mszczodrak/openthread,abtink/openthread,erja-gp/openthread,chshu/openthread,lanyuwen/openthread,srickardti/openthread,librasungirl/openthread,mszczodrak/openthread,abtink/openthread,srickardti/openthread,gandreello/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,georgecpr/openthread,LeZhang2016/openthread,pvanhorn/openthread,bukepo/openthread,georgecpr/openthread,georgecpr/openthread,pvanhorn/openthread,erja-gp/openthread,jwhui/openthread,LeZhang2016/openthread,erja-gp/openthread,chshu/openthread,erja-gp/openthread,turon/openthread,LeZhang2016/openthread,mszczodrak/openthread,abtink/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,georgecpr/openthread,LeZhang2016/openthread,openthread/openthread,bukepo/openthread,abtink/openthread,jwhui/openthread,erja-gp/openthread,mszczodrak/openthread,georgecpr/openthread,librasungirl/openthread,LeZhang2016/openthread,openthread/openthread,openthread/openthread