WASHINGTON — Fifty years after the assassination of Martin Luther King Jr., only 1 in 10 African Americans think the United States has achieved all or most of the goals of the civil rights movement he led, according to a new poll by the Associated Press-NORC Center for Public Affairs Research. Three-quarters of African Americans said there has been little or no progress on fair treatment by police, and more than half answered the same about fair coverage by the media, political representation or equal economic opportunities. Currently, things are steadily “going on a quick downward spiral,” said Stephanie Sutton, 42, a Silver Spring, Maryland, housewife who is black.