Send power hint on fling gestures.

Use the existing PointerEventListener infrastructure to listen for
gestures that look like flings and hint to the power system when they
happen. Since we don't actually have a bound for the fling like a
regular application would, limit them to five seconds and refresh
every time a new fling is seen until the five second time period is
up.

bug 24059298

Change-Id: I5757a1e88f2ab2ef08cccefa8221d809ae71bb6f