Force "not still" for the duration of the window after motion

Because of our motion detection heuristic, we have situations when the
state is considered in motion and then back to still before the window
duration elapses (this is because we always compare the samples in the
buffer to the last one). In order to avoid toggling back-and-forth
faster than the window duration, we add a suppression period after
detecting motion.

Test: atest --host libheadtracking-test
Change-Id: Ia85d5699bef1b3313d007193375df4892f3365ac