[routing-manager] fix prefix comparison in `EvaluateOnLinkPrefix()` (#8192)
[routing-manager] fix prefix comparison in `EvaluateOnLinkPrefix()` (#8192)

This commit fixes `EvaluateOnLinkPrefix()` to use local on-link
prefix (instead of OMR prefix).
openthread/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,srickardti/openthread,jwhui/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread