It’s been two and a half years since PJ Harvey’s last album, the angry, antiwar Let England Shake. Saturday, she released a new song, “Shaker Aamer,” an angry, despairing tribute to the last British resident still held captive at Guantanamo Bay. A 46-year-old citizen of Saudi Arabia with a wife and four children living in London, Shaker Aamer was arrested in November 2001 and sent to the detention camp in Cuba three months later; he’s been there ever since, despite never having been charged with a crime, and having been cleared for release by the Bush administration in 2007, and then again by the Obama administration two years later.