Strength comes in different forms, and two new films with Irish actress Saoirse Ronan tell the tale. In Steve McQueen’s high-octane World War II drama “Blitz,” set in an embattled London, Ronan is Rita, a single mother searching for her young son who’s disappeared from a train ostensibly taking children to safety in the British countryside. As bombs fall and subway stations flood, Rita is just about the fiercest thing in London, abandoning the bomb factory where she works to find her boy at all