Fix status bar brightness control when on secure lock screen

When the notifications panel is disabled no views are handling touch events
on the status bar, so we see nothing beyond each ACTION_DOWN. Intercept touch
events when needed specifically for brightness control.
Also recycle VelocityTracker when we're not using it, as per its documentation.

Change-Id: Ib96eb6a6fd16d086d318136155e16da10600c8fa