Status bar: Make brightness control more responsive

When touching the status bar, screen brightness can be changed after a counter
reaches a threshold. Previously any y-axis movement above a certain velocity
would reset the counter, making it very difficult for the user to smoothly
adjust brightness. With this patch such movements will simply cause the counter
not to increment.

Will need testing to determine whether this makes it too easy to adjust
brightness by mistake.

Change-Id: I3dc9185f0240b58b59dd0b641f96f8cf8e97ed9f