AP Medical Writer WASHINGTON -- At least 120,000 Americans a year suffer strokes because of a common irregular heartbeat -- one that's on the rise, hard to treat and can shoot deadly blood clots straight to the brain. Now doctors are experimenting with a new way to prevent those brain attacks: a tiny device that seals off a little section of the jiggling heart where the clots form. If it works -- and a major study is under way -- the Watchman device might provide long-needed protection for thousands of people with atrial fibrillation, whose main hope now is a...