“May you live in interesting times.” So goes the famous Chinese saying. And the times couldn’t be more interesting for “The Great Wall” — the biggest, most expensive movie ever made in China — arriving in American theaters at a point when U.S.-China relations could scarcely be more consequential, especially when it comes to movies. A bit of history: For the past several years, Hollywood and China have been engaged in a wary dance that could be both lucrative or disastrous, depending on what’s at stake.