[notifier] add string for flags newly introduced (#4820)
[notifier] add string for flags newly introduced (#4820)

- OT_CHANGED_THREAD_BACKBONE_ROUTER_STATE
- OT_CHANGED_THREAD_BACKBONE_ROUTER_LOCAL
- OT_CHANGED_JOINER_STATE
lanyuwen/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,bukepo/openthread,srickardti/openthread,lanyuwen/openthread,librasungirl/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,librasungirl/openthread,abtink/openthread,openthread/openthread,lanyuwen/openthread,librasungirl/openthread,bukepo/openthread,abtink/openthread,bukepo/openthread,openthread/openthread,chshu/openthread,openthread/openthread,srickardti/openthread,chshu/openthread,jwhui/openthread,bukepo/openthread,jwhui/openthread,abtink/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,srickardti/openthread