Make default brightness ramp rates multiples of 60.

If ramp rates aren't even multiple of 60 then we're more likely to get
brightness changes that differ each frame, which can show up as a
stuttering in the animation.

Bug: 64514692
Test: made change, took systrace, saw smooth ramping
Change-Id: Iada2f1de684d94fe790ab8d5cf03bea5ba1a2ca5