[thci] fix implementation of `setKeepAliveTimeOut` (#7373)
[thci] fix implementation of `setKeepAliveTimeOut` (#7373)

Thread 1.2 harness uses this method to set the child timeout in
DUA-TC-10, DUA-TC-21 and DUA-TC-22.
openthread/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,srickardti/openthread