Fix a math error in StillnessDetector::areNear()

The method used to determine the angular distance has been outright
wrong. Using a slightly more expensive, but accurate method instead.

Tightened up the test.

Test: atest --host libheadtracking-test:StillnessDetectorTest --rerun-until-failure 1000
Change-Id: I0f6b01340b6666b95ad032da8cd09ac1a33a8b41